This normal thing one to happen especially if u got using water heater. Even u got exhaust fan and small windows that keep open, the wap from hot water 1st place it go is up ur ceiling, so ur ceiling is 1st to absorb the wap, so it will slowly cause mold to appear. Normal, everything need maintenance. Unless ur ceiling material is waterproof hard plywood wif good paint n no need plaster, then myb ok.

Yes usually people said use anti-mold paint can solve but not really solving it, after few months comes again. My fren lastime ever try paint his bathroom ceiling using good weathershield paint, so far so good he said, but no more update after tat. U try look for dulux Weathershield Mould.