I noticed that Mozart was appearing as a production credit in my library. It turned out to be a single album in the collection:
I first went to edit the credits of the album, and removed Wolfgang Amadeus Mozart from being a production credit. That seemed to have no effect - Mozart still had a production credit in Roon.
Then I selected all the tracks in the album, went to the track editor and removed Mozart from here.
It’s inconvenient from a credit-editing perspective, but I would expect that adding a single production credit at the track or album level would result in the individual gaining Performer status.
In other words, adding a credit works for both levels, removing a credit does not. This probably needs to be spelled out in the User Guide. It seems to me that these sort of quirks just add weight to the call for a review of the UI design and experience.