@Ivan, I think I may have found the source of the problem, which generally speaking, may be Roon’s renewed attempt to impose a genre scheme (see my other note on this at Roon 1.8 problem: Has Roon decided to impose a genre scheme on users again?).
In my book, the only “Classical” composers represented in my collection who are strictly speaking classical composers are Haydn, Herschel, and Mozart. These are the only three in my collection whose compositions are indicated as being “Classical” in the GENRE tag of my source files. All other loosely speaking “Classical” composers in my database write compositions that are are either “Early”, “Renaissance”, “Baroque”, “Romantic”, or “Modern” as far as the GENRE tag of the source files is concerned. (Gesualdo, for example, writes “Renaissance” compositions.) All of these composers, however, regardless of the GENRE tag in the sources for their compositions, are designated as “Is Classical Composer: Yes” in the composer edit fields of my database.
I just checked my database again and sure enough, Haydn, Herschel, and Mozart seem to be the only composers in my collection for whom the “Composed By” display seems to be working properly. I will do some experimentation in the morning with my file GENRE tags in the morning to explore this hypothesis. If you have a way to check, could you look into whether the Roon is not only using the “Is Classical Composer” checkbox in constructing “Composed By”, but also the GENRE tags of the pertinent compositions? In my opinion doing the former is fine, doing the latter is not (ref: the note on Roon imposing Genres on us above).