Backing store is not deleted after visible native child widget is reparented

QWidget parent1;
parent1.setGeometry(50, 50, 100, 100);

QWidget parent2;
parent2.setGeometry(150, 150, 100, 100);

QWidget child(&parent1);
child.winId();
child.setGeometry(10, 10, 30, 30);

parent1.show();

// At this point, parent1's backing store has a reference count of 2 (parent1 + child)

parent2.show();

// parent2's backing store has a reference count of 1 (parent2)

child.setParent(&parent2);
child.show();

// Now parent2's backing store has a reference count of 2 (parent2 + child)

parent1.hide();

// parent1's backing store's reference count should now have gone to zero and it should therefore be deleted, but this does not happen