Fixes as follows:

src/Storage/DbalNestedSet.php

@@ -211,7 +211,7 @@ public function getTree() {
     $tree = [];
     $stmt = $this->connection->executeQuery('SELECT id, revision_id, left_pos, right_pos, depth FROM ' . $this->tableName . ' ORDER BY left_pos');
     while ($row = $stmt->fetch()) {
-      $tree[] = new Node($row['id'], $row['revision_id'], $row['left_pos'], $row['right_pos'], $row['depth']);
+      $tree[] = new Node(new NodeKey($row['id'], $row['revision_id']), $row['left_pos'], $row['right_pos'], $row['depth']);
     }
     return $tree;
   }
@@ -308,23 +308,23 @@ public function moveSubTreeToRoot(Node $node) {
    */
   public function moveSubTreeBelow(Node $target, Node $node) {
     $newLeftPosition = $target->getLeft() + 1;
-    $this->moveSubTreeToPosition($newLeftPosition, $node);
+    $this->moveSubTreeToPosition($newLeftPosition, $node, $target->getDepth() + 1);
   }
 
   /**
    * {@inheritdoc}
    */
   public function moveSubTreeBefore(Node $target, Node $node) {
     $newLeftPosition = $target->getLeft();
-    $this->moveSubTreeToPosition($newLeftPosition, $node);
+    $this->moveSubTreeToPosition($newLeftPosition, $node, $target->getDepth());
   }
 
   /**
    * {@inheritdoc}
    */
   public function moveSubTreeAfter(Node $target, Node $node) {
     $newLeftPosition = $target->getRight() + 1;
-    $this->moveSubTreeToPosition($newLeftPosition, $node);
+    $this->moveSubTreeToPosition($newLeftPosition, $node, $target->getDepth());
   }
 
   /**
@@ -334,11 +334,13 @@ public function moveSubTreeAfter(Node $target, Node $node) {
    *   The new left position.
    * @param \PNX\NestedSet\Node $node
    *   The node to move.
+   * @param int $newDepth
+   *   Depth of new position.
    *
    * @throws \Exception
    *   If a transaction error occurs.
    */
-  protected function moveSubTreeToPosition($newLeftPosition, Node $node) {
+  protected function moveSubTreeToPosition($newLeftPosition, Node $node, $newDepth) {
     try {
       // Calculate position adjustment variables.
       $width = $node->getRight() - $node->getLeft() + 1;
@@ -348,8 +350,7 @@ protected function moveSubTreeToPosition($newLeftPosition, Node $node) {
       $this->connection->beginTransaction();
 
       // Calculate depth difference.
-      $newNode = $this->getNodeAtPosition($newLeftPosition);
-      $depthDiff = $newNode->getDepth() - $node->getDepth();
+      $depthDiff = $newDepth - $node->getDepth();
 
       // Backwards movement must account for new space.
       if ($distance < 0) {

tests/Functional/DbalNestedSetTest.php

@@ -346,6 +346,81 @@ public function testMoveSubTreeBelow() {
 
   }
 
+  /**
+   * Tests moving a sub-tree under a brand new parent node.
+   */
+  public function testMoveSubTreeBelowEndParentNode() {
+
+    $parent = $this->nestedSet->getNode(new NodeKey(1, 1));
+    $nodeKey = new NodeKey(7, 1);
+    $node = $this->nestedSet->getNode($nodeKey);
+
+    $newRoot = $this->nestedSet->addRootNode(new NodeKey(12, 1));
+    $this->nestedSet->moveSubTreeBelow($newRoot, $node);
+
+    // Check node is in new position.
+    $node = $this->nestedSet->getNode($nodeKey);
+    $this->assertEquals(18, $node->getLeft());
+    $this->assertEquals(23, $node->getRight());
+    $this->assertEquals(1, $node->getDepth());
+
+    // Check children are in new position.
+    $node = $this->nestedSet->getNode(new NodeKey(10, 1));
+    $this->assertEquals(19, $node->getLeft());
+    $this->assertEquals(20, $node->getRight());
+    $this->assertEquals(2, $node->getDepth());
+
+    $node = $this->nestedSet->getNode(new NodeKey(11, 1));
+    $this->assertEquals(21, $node->getLeft());
+    $this->assertEquals(22, $node->getRight());
+    $this->assertEquals(2, $node->getDepth());
+
+    // Check old parent is updated.
+    $node = $this->nestedSet->getNode(new NodeKey(3, 1));
+    $this->assertEquals(10, $node->getLeft());
+    $this->assertEquals(15, $node->getRight());
+    $this->assertEquals(1, $node->getDepth());
+
+  }
+
+  /**
+   * Tests moving a sub-tree under a brand new child node.
+   */
+  public function testMoveSubTreeBelowEndChildNode() {
+
+    $parent = $this->nestedSet->getNode(new NodeKey(1, 1));
+    $nodeKey = new NodeKey(7, 1);
+    $node = $this->nestedSet->getNode($nodeKey);
+
+    $newRoot = $this->nestedSet->addRootNode(new NodeKey(12, 1));
+    $newChild = $this->nestedSet->addNodeBelow($newRoot, new NodeKey(13, 1));
+    $this->nestedSet->moveSubTreeBelow($newChild, $node);
+
+    // Check node is in new position.
+    $node = $this->nestedSet->getNode($nodeKey);
+    $this->assertEquals(19, $node->getLeft());
+    $this->assertEquals(24, $node->getRight());
+    $this->assertEquals(2, $node->getDepth());
+
+    // Check children are in new position.
+    $node = $this->nestedSet->getNode(new NodeKey(10, 1));
+    $this->assertEquals(20, $node->getLeft());
+    $this->assertEquals(21, $node->getRight());
+    $this->assertEquals(3, $node->getDepth());
+
+    $node = $this->nestedSet->getNode(new NodeKey(11, 1));
+    $this->assertEquals(22, $node->getLeft());
+    $this->assertEquals(23, $node->getRight());
+    $this->assertEquals(3, $node->getDepth());
+
+    // Check old parent is updated.
+    $node = $this->nestedSet->getNode(new NodeKey(3, 1));
+    $this->assertEquals(10, $node->getLeft());
+    $this->assertEquals(15, $node->getRight());
+    $this->assertEquals(1, $node->getDepth());
+
+  }
+
   /**
    * Tests moving a sub-tree to the root.
    */
@@ -461,6 +536,13 @@ public function testValidateTableInvalidFirstChars() {
     $this->nestedSet = new DbalNestedSet($this->connection, "1abc");
   }
 
+  /**
+   * Test get tree works.
+   */
+  public function testGetTree() {
+    $this->assertCount(11, $this->nestedSet->getTree());
+  }
+
   /**
    * Loads the test data.
    */