In my current game (no save available, sorry), I've been suzerain of Nan Madol for most of the game. Any districts I build on the coast are getting the +2 culture, but my city centers on the coast are not. I swear that the bonus used to apply to city centers also (and it makes sense, since they are treated as districts for adjacencies, etc.). Am I just misremembering, is this a bug, or was this something changed?