fix

app/code/Magento/Customer/Model/Plugin/UpdateCustomerId.php

@@ -0,0 +1,49 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+
+declare(strict_types=1);
+
+namespace Magento\Customer\Model\Plugin;
+
+use Magento\Framework\Webapi\Rest\Request as RestRequest;
+use Magento\Customer\Api\Data\CustomerInterface;
+use Magento\Customer\Api\CustomerRepositoryInterface;
+
+/**
+ * Update customer id from request param
+ */
+class UpdateCustomerId
+{
+    /**
+     * @var RestRequest $request
+     */
+    private $request;
+
+    /**
+     * @param RestRequest $request
+     */
+    public function __construct(RestRequest $request)
+    {
+        $this->request = $request;
+    }
+
+    /**
+     * Update customer id from request if exist
+     *
+     * @param CustomerRepositoryInterface $customerRepository
+     * @param CustomerInterface $customer
+     * @return void
+     * @SuppressWarnings(PHPMD.UnusedFormalParameter)
+     */
+    public function beforeSave(CustomerRepositoryInterface $customerRepository, CustomerInterface $customer): void
+    {
+        $cartId = $this->request->getParam('customerId');
+
+        if ($cartId) {
+            $customer->setId($cartId);
+        }
+    }
+}

app/code/Magento/Customer/Model/ResourceModel/CustomerRepository.php

@@ -26,6 +26,7 @@
 use Magento\Framework\Api\SearchCriteria\CollectionProcessorInterface;
 use Magento\Framework\Api\SearchCriteriaInterface;
 use Magento\Framework\App\ObjectManager;
+use Magento\Framework\EntityManager\HydratorInterface;
 use Magento\Framework\Event\ManagerInterface;
 use Magento\Store\Model\StoreManagerInterface;
 
@@ -119,6 +120,11 @@ class CustomerRepository implements CustomerRepositoryInterface
      */
     private $delegatedStorage;
 
+    /**
+     * @var HydratorInterface
+     */
+    private $hydrator;
+
     /**
      * @param CustomerFactory $customerFactory
      * @param CustomerSecureFactory $customerSecureFactory
@@ -136,6 +142,7 @@ class CustomerRepository implements CustomerRepositoryInterface
      * @param CollectionProcessorInterface $collectionProcessor
      * @param NotificationStorage $notificationStorage
      * @param DelegatedStorage|null $delegatedStorage
+     * @param HydratorInterface|null $hydrator
      * @SuppressWarnings(PHPMD.ExcessiveParameterList)
      */
     public function __construct(
@@ -154,7 +161,8 @@ public function __construct(
         JoinProcessorInterface $extensionAttributesJoinProcessor,
         CollectionProcessorInterface $collectionProcessor,
         NotificationStorage $notificationStorage,
-        DelegatedStorage $delegatedStorage = null
+        DelegatedStorage $delegatedStorage = null,
+        ?HydratorInterface $hydrator = null
     ) {
         $this->customerFactory = $customerFactory;
         $this->customerSecureFactory = $customerSecureFactory;
@@ -172,6 +180,7 @@ public function __construct(
         $this->collectionProcessor = $collectionProcessor;
         $this->notificationStorage = $notificationStorage;
         $this->delegatedStorage = $delegatedStorage ?? ObjectManager::getInstance()->get(DelegatedStorage::class);
+        $this->hydrator = $hydrator ?: ObjectManager::getInstance()->get(HydratorInterface::class);
     }
 
     /**
@@ -185,6 +194,7 @@ public function __construct(
      * @throws \Magento\Framework\Exception\LocalizedException
      * @SuppressWarnings(PHPMD.CyclomaticComplexity)
      * @SuppressWarnings(PHPMD.NPathComplexity)
+     * @SuppressWarnings(PHPMD.ExcessiveMethodLength)
      */
     public function save(CustomerInterface $customer, $passwordHash = null)
     {
@@ -193,10 +203,11 @@ public function save(CustomerInterface $customer, $passwordHash = null)
         $prevCustomerData = $prevCustomerDataArr = null;
         if ($customer->getId()) {
             $prevCustomerData = $this->getById($customer->getId());
-            $prevCustomerDataArr = $prevCustomerData->__toArray();
+            $prevCustomerDataArr = $this->hydrator->extract($prevCustomerData);
+            $customer = $this->hydrator->hydrate($prevCustomerData, $customer->__toArray());
         }
         /** @var $customer \Magento\Customer\Model\Data\Customer */
-        $customerArr = $customer->__toArray();
+        $customerArr = $this->hydrator->extract($customer);
         $customer = $this->imageProcessor->save(
             $customer,
             CustomerMetadataInterface::ENTITY_TYPE_CUSTOMER,