Could it be the inverse of the Dolly Parton bug reported here:?
Can you recall what Mozart releases you addded recently? If so, I might try removing one or two from my library in case it was some bad metadata that causes Roon to fail, rather like it did for Dolly Parton in my case. Or perhaps you have the “Classical only” switch selected, and Roon in its infinite wisdom has decided that Mozart is no longer classical.
And, just for the record, Mozart appears not only as a composer for me, but as a performer and producer.