Had a similar problem with several classical composers who are long dead but appear both as a composer and a performer (Richard Strauss, Benjamin Britten and others). If I recall it correctly, removing ´Rachmaninov´ from all other roles than composer and primary artist, solved the issue. Reason being that roon files albums on the artist´s and composer´s page either under ´composed by´, ´main albums´ or ´appearances´, not several at the same time. So if one is both a composer and ´Soloist´, ´Piano´ in your case, the album does not appear as classical composers before the age of mass recordings seemingly do not have an ´appearances´ section.
Make sure you edit the credits correctly both for all tracks of the album containing any Rachmaninov credit other than ´composer´ and for the album itself.
Does the album in question show up in album overview when focus-filtering for ´rachmaninov´ as a composer or a performer?
Should not be a problem here, as the original spelling of Рахманинов is in the database, and all Cyrillic letters contained in this name, do offer a clear transliteration. There are just different conventions of transliterating, particularly for the Russian ´в´ which can be ´v´ in latin letters, or ´w´ or ´ff´.
Roon understands different transliterations well in my experience, maybe except some non-standard ways of transliterating sounds in Russian which do simply not have any equivalent in Latin alphabet, like ´җ´ or ´ Щ´.