With a particular multiscreen setup that is used a lot especially in laptops, the following happens:
if the system is configured to automatically disable the internal display when connecting a external one, so that the laptop continues to use a single screen when the external one is connected, the QScreen * pointer returned by QGuiApplication::primaryScreen() is recycled to represent the new screen (geometry changed signals will be emitted if needed)
so no QGuiApplication::primaryScreenChanged() signal gets emitted (neither screenAdded/screenRemoved), making impossible for a client application to correctly notice the screen setup actually changed.