Thanks for your efforts.
As you said it works in a project I have a second look at my code.
I'm not writing a Quick-App but a QWidget-App and use the map inside a QQuickWidget. I have read about another approach to embed qml content in a QWidget-App using QQuickView inside a QObject-Container.
This approach works as expected. I don't know why it doesn't work with a QQuickWidget but my problem is solved.