You can ask suno for a style, so you could put "country anthem", "serious", "orchestral", etc, for less pop music stuff. Or you could select for very cursed things.
Since there are so little national anthems due to fact there are like only few hundred nations including Countries that no longer exist. It just treats word "Anthem" as Like Party rock its trained for Pop songs. maybe that's why it sounds like a Bieber song.