Probably not a bug no, there is a precedent: Both a fully formed HRE and Germany are Germanic cultural unions, and nothing prevents the two countries to exist at the same time (albeit it can be difficult to do).
Therefore, Russia and Ruthenia could both be the Cultural union of West Slavic, with Ruthenia more centered on Ruthenian/Byelorussian lands and accepting Russians when Russia is centered on Russian lands and accept the two other cultures. Nothing wrong with that and it makes the game interesting.