If you move an item from one level to another into the tree, the indent of the corresponding row won't get updated. This is reproducible only if the new parent of the moved item is expanded, while the operation is executed.
Usually, this can be reproduced by calling the ExpandHierarchyItem() for the new parent item method just after the moving of the original item.
To resolve this you can delay the expanding of the item. Note that the expanding is executed manually by calling the ExpandHierarchyItem() method. So, you can use a dispatcher with a lower DispatcherPriority to delay this.
Or alternatively, you can collapse the item before expanding it (using the CollapseHierarchyItem() method).