If we set faction creator to something like parthia for a seleucid territory, the buildings we put in it will still default to that greek culture variant instead of the eastern one if there are multiple descriptions provided.

But these provinces have greek_cities as the possessor at game's start and also greek_cities as the faction_creator in descr_strat.txt and greek_cities as the default culture in the regions.txt file. And there is a greek_cities description, enum, and entry in the building complex itself too. That's what is so puzzling. And it is happening with multiple complexes too, not just one.

The same thing had occurred with the macedonian building before - it said (ROMAN) but it was a macedonian held, created, and cultured province, with macedonian variants in description, enum, and complex entries.

I wonder if it has something to do with the fact that all greek buildings are technically "inside" of the roman tree for building graphics. They will use roman versions if no greek one is provided. This might be true of all cultures, if no other variant is provided though, but I can't speak for sure on that.