Calling QQuickWindow::setTransientParent does not take effect

Using the transientParent property like following works as expected:

Window {
    transientParent: mainwindow
}

But once I move that assignment into onCompleted signal, it no longer works. It may be that this only works before the window is created, but I could not find that information in the documentation. I tried assigning the transient parent using a C++ method as well. And That doesn't work either.

Is it not possible to change the transient window or is this a bug?