diff --git a/typo3/sysext/backend/Classes/Controller/FormSelectTreeAjaxController.php b/typo3/sysext/backend/Classes/Controller/FormSelectTreeAjaxController.php
index 2b5715bfb95d..a75dd0b5a0a7 100644
--- a/typo3/sysext/backend/Classes/Controller/FormSelectTreeAjaxController.php
+++ b/typo3/sysext/backend/Classes/Controller/FormSelectTreeAjaxController.php
@@ -103,41 +103,40 @@ public function fetchDataAction(ServerRequestInterface $request): ResponseInterf
                         ],
                     ];
                 }
-            } else {
-                if (isset($dataStructure['sheets'][$flexFormSheetName]['ROOT']
-                    ['el'][$flexFormFieldName]
-                    ['el'][$flexFormContainerName]
-                    ['el'][$flexFormContainerFieldName])
-                ) {
-                    // If this is a tree in a section container that has just been added by the FlexFormAjaxController
-                    // "new container" action, then this container is not yet persisted, so we need to trigger the
-                    // TcaFlexProcess data provider again to prepare the DS and databaseRow of that container.
-                    if ($flexFormSectionContainerIsNew) {
-                        $flexSectionContainerPreparation = [
-                            'flexFormSheetName' => $flexFormSheetName,
-                            'flexFormFieldName' => $flexFormFieldName,
-                            'flexFormContainerName' => $flexFormContainerName,
-                            'flexFormContainerIdentifier' => $flexFormContainerIdentifier,
-                        ];
-                    }
-                    // Now restrict the data structure to our tree element only
-                    $dataStructure = [
-                        'sheets' => [
-                            $flexFormSheetName => [
-                                'ROOT' => [
-                                    'type' => 'array',
-                                    'el' => [
-                                        $flexFormFieldName => [
-                                            'section' => 1,
-                                            'type' => 'array',
-                                            'el' => [
-                                                $flexFormContainerName => [
-                                                    'el' => [
-                                                        $flexFormContainerFieldName => $dataStructure['sheets'][$flexFormSheetName]['ROOT']
-                                                            ['el'][$flexFormFieldName]
-                                                            ['el'][$flexFormContainerName]
-                                                            ['el'][$flexFormContainerFieldName]
-                                                    ],
+            } elseif (isset($dataStructure['sheets'][$flexFormSheetName]['ROOT']
+                ['el'][$flexFormFieldName]
+                ['el'][$flexFormContainerName]
+                ['el'][$flexFormContainerFieldName])
+            ) {
+                // If this is a tree in a section container that has just been added by the FlexFormAjaxController
+                // "new container" action, then this container is not yet persisted, so we need to trigger the
+                // TcaFlexProcess data provider again to prepare the DS and databaseRow of that container.
+                if ($flexFormSectionContainerIsNew) {
+                    $flexSectionContainerPreparation = [
+                        'flexFormSheetName' => $flexFormSheetName,
+                        'flexFormFieldName' => $flexFormFieldName,
+                        'flexFormContainerName' => $flexFormContainerName,
+                        'flexFormContainerIdentifier' => $flexFormContainerIdentifier,
+                    ];
+                }
+                // Now restrict the data structure to our tree element only
+                $dataStructure = [
+                    'sheets' => [
+                        $flexFormSheetName => [
+                            'ROOT' => [
+                                'type' => 'array',
+                                'el' => [
+                                    $flexFormFieldName => [
+                                        'section' => 1,
+                                        'type' => 'array',
+                                        'el' => [
+                                            $flexFormContainerName => [
+                                                'type' => 'array',
+                                                'el' => [
+                                                    $flexFormContainerFieldName => $dataStructure['sheets'][$flexFormSheetName]['ROOT']
+                                                        ['el'][$flexFormFieldName]
+                                                        ['el'][$flexFormContainerName]
+                                                        ['el'][$flexFormContainerFieldName]
                                                 ],
                                             ],
                                         ],
@@ -145,8 +144,8 @@ public function fetchDataAction(ServerRequestInterface $request): ResponseInterf
                                 ],
                             ],
                         ],
-                    ];
-                }
+                    ],
+                ];
             }
             $processedTca['columns'][$fieldName]['config']['ds'] = $dataStructure;
             $processedTca['columns'][$fieldName]['config']['dataStructureIdentifier'] = $dataStructureIdentifier;
diff --git a/typo3/sysext/backend/Tests/Unit/Form/FormDataProvider/TcaFlexPrepareTest.php b/typo3/sysext/backend/Tests/Unit/Form/FormDataProvider/TcaFlexPrepareTest.php
index bc7d05ab69a3..1d9593365e21 100644
--- a/typo3/sysext/backend/Tests/Unit/Form/FormDataProvider/TcaFlexPrepareTest.php
+++ b/typo3/sysext/backend/Tests/Unit/Form/FormDataProvider/TcaFlexPrepareTest.php
@@ -285,4 +285,224 @@ public function addDataInitializesDatabaseRowValueIfNoDataStringIsGiven()
 
         self::assertEquals($expected, (new TcaFlexPrepare())->addData($input));
     }
+
+    /**
+     * @test
+     */
+    public function addDataSetsParsedDataStructureArrayRecursive(): void
+    {
+        $input = [
+            'systemLanguageRows' => [],
+            'tableName' => 'aTableName',
+            'databaseRow' => [
+                'aField' => [
+                    'data' => [],
+                    'meta' => [],
+                ],
+            ],
+            'processedTca' => [
+                'columns' => [
+                    'aField' => [
+                        'config' => [
+                            'type' => 'flex',
+                            'ds' => [
+                                'default' => '
+                                    <T3DataStructure>
+                                        <sheets>
+                                            <sTree>
+                                                <ROOT>
+                                                    <type>array</type>
+                                                    <TCEforms>
+                                                        <sheetTitle>selectTree</sheetTitle>
+                                                    </TCEforms>
+                                                    <el>
+                                                        <select_tree_1>
+                                                            <TCEforms>
+                                                                <label>select_tree_1</label>
+                                                                <description>select_tree_1 description</description>
+                                                                <config>
+                                                                    <type>select</type>
+                                                                    <renderType>selectTree</renderType>
+                                                                </config>
+                                                            </TCEforms>
+                                                        </select_tree_1>
+                                                    </el>
+                                                </ROOT>
+                                            </sTree>
+                                            <sSection>
+                                                <ROOT>
+                                                    <type>array</type>
+                                                    <TCEforms>
+                                                        <sheetTitle>section</sheetTitle>
+                                                    </TCEforms>
+                                                    <el>
+                                                        <section_1>
+                                                            <title>section_1</title>
+                                                            <type>array</type>
+                                                            <section>1</section>
+                                                            <el>
+                                                                <container_1>
+                                                                    <type>array</type>
+                                                                    <title>container_1</title>
+                                                                    <el>
+                                                                        <select_tree_2>
+                                                                            <TCEforms>
+                                                                                <label>select_tree_2</label>
+                                                                                <description>select_tree_2 description</description>
+                                                                                <config>
+                                                                                    <type>select</type>
+                                                                                    <renderType>selectTree</renderType>
+                                                                                </config>
+                                                                            </TCEforms>
+                                                                        </select_tree_2>
+                                                                    </el>
+                                                                </container_1>
+                                                            </el>
+                                                        </section_1>
+                                                    </el>
+                                                </ROOT>
+                                            </sSection>
+                                        </sheets>
+                                    </T3DataStructure>
+                                ',
+                            ],
+                        ],
+                    ],
+                ],
+            ],
+        ];
+
+        $GLOBALS['TCA']['aTableName']['columns'] = $input['processedTca']['columns'];
+
+        $expected = $input;
+        $expected['processedTca']['columns']['aField']['config']['dataStructureIdentifier']
+            = '{"type":"tca","tableName":"aTableName","fieldName":"aField","dataStructureKey":"default"}';
+
+        $expected['processedTca']['columns']['aField']['config']['ds'] = [
+            'sheets' => [
+                'sSection' => [
+                    'ROOT' => [
+                        'type' => 'array',
+                        'sheetTitle' => 'section',
+                        'el' => [
+                            'section_1' => [
+                                'title' => 'section_1',
+                                'type' => 'array',
+                                'section' => '1',
+                                'el' => [
+                                    'container_1' => [
+                                        'type' => 'array',
+                                        'title' => 'container_1',
+                                        'el' => [
+                                            'select_tree_2' => [
+                                                'label' => 'select_tree_2',
+                                                'description' => 'select_tree_2 description',
+                                                'config' => [
+                                                    'type' => 'select',
+                                                    'renderType' => 'selectTree'
+                                                ]
+                                            ]
+                                        ]
+                                    ]
+                                ]
+                            ]
+                        ],
+                    ],
+                ],
+                'sTree' => [
+                    'ROOT' => [
+                        'type' => 'array',
+                        'sheetTitle' => 'selectTree',
+                        'el' => [
+                            'select_tree_1' => [
+                                'label' => 'select_tree_1',
+                                'description' => 'select_tree_1 description',
+                                'config' => [
+                                    'type' => 'select',
+                                    'renderType' => 'selectTree'
+                                ]
+                            ]
+                        ]
+                    ]
+                ],
+            ],
+            'meta' => [],
+        ];
+
+        self::assertEquals($expected, (new TcaFlexPrepare())->addData($input));
+    }
+
+    /**
+     * Test of the data provider when called for a section with already
+     * resolved flex form, e.g. in an ajax request (tcaSelectTreeAjaxFieldData),
+     * which got "reduced to the relevant element only".
+     *
+     * @test
+     */
+    public function addDataMigratesResolvedFlexformTca(): void
+    {
+        $columnConfig = [
+            'label' => 'select_section_1',
+            'description' => 'select_section_1 description',
+            'config' => [
+                'type' => 'select'
+            ]
+        ];
+
+        $input = [
+            'systemLanguageRows' => [],
+            'tableName' => 'aTableName',
+            'databaseRow' => [
+                'aField' => [
+                    'data' => [],
+                    'meta' => [],
+                ],
+            ],
+            'processedTca' => [
+                'columns' => [
+                    'aField' => [
+                        'config' => [
+                            'type' => 'flex',
+                            'ds' => [
+                                'sheets' => [
+                                    'sSection' => [
+                                        'ROOT' => [
+                                            'type' => 'array',
+                                            'el' => [
+                                                'section_1' => [
+                                                    'section' => 1,
+                                                    'type' => 'array',
+                                                    'el' => [
+                                                        'container_1' => [
+                                                            'type' => 'array',
+                                                            'el' => [
+                                                                'select_section_1' => [
+                                                                    'TCEforms' => $columnConfig
+                                                                ]
+                                                            ]
+                                                        ]
+                                                    ]
+                                                ]
+                                            ]
+                                        ]
+                                    ]
+                                ]
+                            ],
+                            'dataStructureIdentifier' => '{"type":"tca","tableName":"aTableName","fieldName":"aField","dataStructureKey":"default"}'
+                        ],
+                    ],
+                ],
+            ],
+        ];
+
+        $expected = $input;
+        $expected['processedTca']['columns']['aField']['config']['ds']['meta'] = [];
+        $expected['processedTca']['columns']['aField']['config']['ds']
+            ['sheets']['sSection']['ROOT']['el']
+                ['section_1']['el']
+                    ['container_1']['el']
+                        ['select_section_1'] = $columnConfig;
+
+        self::assertEquals($expected, (new TcaFlexPrepare())->addData($input));
+    }
 }