B.C.
We have a tiny bathroom in a corner of the house that also has this issue.
Repeated cleaning and bleaching didn't resolve it.
Somehow, it seems the fan vent allows a back flow of moist air when it's not running (which is most of the time - we're not running the fan 24/7 when the room is not being used - that's just a waste of electricity) and it's always very humid here spring through fall.
So I got some Damp Rid (the kind that hangs in a closet) and we hang it on the inside door knob (no one sees it unless they close the door) and TA DA! - no more mildew!