They get written there every ban. However, if you don't have 'exec banned_users.cfg' in your server.cfg file... your ban list gets cleared every server restart, because the writeid that ULX performs writes empty data if you haven't.
This isn't a ULX problem.. this is how Source works. You're supposed to load your banned users on server/map startup before you write to it again or its cleared because nothing was in it to writeid