Hi @Klaus_Kammerer1. I think I know what is going on here and I’ve already (hopefully) put in a fix.
This seems to be arising from a bad MusicBrainz recording credit. You can see the edit that I have submitted here. Someone mistakenly credited Dresden Kreuzchor as “Felix Mendelssohn Bartholdy”.
Mendelssohn and Dresden Kreuzchor are completely separate in our Cloud metadata, but Roon attempts additional equivalence with your textual track tags locally. In general this works well, but in this case the client sees “Felix Mendelssohn Bartholdy” as an alias of Dresden Kreuzchor and does some incorrect matching. When an alias is completely wrong, some strange and incomprehensible equivalences can happen.
It may take up to a week for this fix to be approved in MusicBrainz (though there is a bug in MusicBrainz which may mean that the unapproved fix might come in sooner), and then up to a week for the updated Dresden Kreuzchor alias list to be pulled into your library. When this happens, the problem should go away automatically as your Core reprocesses anything related to Dresden Kreuzchor.
I appreciate your frustration / hair pulling, but this should be resolved in a week or two. Hang in there and thanks for bringing this to our attention!