switching from fullscreen to windowed mode not working

Hello,

i have a Window which is configured to be fullscreen.

Window
{
    visible                 : true
    visibility              : Window.FullScreen

...

}

if the CommandLineParser-Option "window" is set i want to change to windowed mode.

    engine->load(QUrl(QStringLiteral("qrc:/main.qml")));

    if (engine->rootObjects().isEmpty())
        return false;

    if (true == parser.isSet("window"))
    {
        engine->rootObjects().first()->setProperty("visibility", QWindow::Windowed);
    }

When i do so the application window switches back to fullscreen magically. The debug output in the window.onVisibilityChanged handler says:

qml: window running in fullscreen mode
qml: window running in windowed mode
qml: window running in fullscreen mode

Very strange - why is that ????