Setting transient parent for child window does not work when using QQuickWidget

The following code creates two windows where one is the transientParent of the other

import QtQuick

Window {
    Rectangle {
        id: root

        color: "red"
        width: 500
        height: 500

        property Window w: Window.window

        Window {
            visible:  true
            transientParent: root.w
        }
    }
}
 

It works as expected.

When replacing the root window with a QMainWindow+QQuickWidget it stops working. See https://invent.kde.org/nicolasfella/qquickwidgetparenttest for the code.

Running it I get

> qrc:/main.qml:11:5: Unable to assign QQuickWidgetOffscreenWindow to QQuickWindowQmlImpl

Replacing "property Window" with "property var" makes the warning go away, but the windows still have no transient parent relationship.

Tested with Qt 6.7 (qtbase 9539f527fcda6ca9b8df09e63fd93f0206649a41, qtdeclarative baa0f7b55fc85e1db0debbce5820aa42ac10c088) and 6.6.2