Looks like I found something.
In the Android ‘Settings’ I selected ‘Battery->Battery optimization’, and then from the pull down in the upper left corner I selected ‘All apps’, then I scrolled down to the Roon app and set it to ‘Not optimized’.
With this changed setting I played music for 1 hour without a restart of the app or a lost connection, I never had this before!
Still I consider this a workaround because the 8 days battery life will be gone by now. Would be nice if development can have a look why the app isn’t compatible with battery optimization.