I adapted the example app to support creating windows and embedding content into them (see attachment).
When C++ clients (single threaded) are connecting everything works fine. I am also able to embed the QtCreator etc. without problems via using --platform wayland. But when a qml client (+render thread) is connected, the event loop of the client freezes (see stacktrace.txt) and the client is not responsive nor timer etc. are working. Embedding any arbitrary qml file via qmlscenes is also not possible.
It seems that the client gets stuck after an expose event from the main thread, while the render thread tries a frame sync. I can imagine, that the wayland compositor is somehow blocking it, and the render thread is blocking the main thread. But as you can see in the source, its nothing special.