When the contents property is aliased to be the data of another item then children which are added via C++ are not handled correctly. If a child item is added via QML then it ends up associated with the aliased item. But if it is added via C++ (using QQmlComponent to create) then it stays with the given parent instead of being passed on as expected.

In the example, you should see the text item appear at the top of the inner rectangle, similar to how the other one is in the bottom of it.