I am afraid, i switched to LineageOS which makes it possible to use Android 11, which solved the issue. There are two threads which confirm that it’s a problem with Android 9.0:
https://community.roonlabs.com/t/roon-arc-stops-playing-on-qobuz-tidal-lossy-formats-balanced-balanced-optimized/
Sorry, cant reproduce it anymore. But now Roon Remote crashes while playing on LineageOS 18.1, i will retrieve the debug logs via USB from the phone and start a new bug report in the corresponding subforum.