This is assuming the exhaust fan is vented to the outside, and not just
into the attic...
It's odd that the flat top coat would blister, unless it's over a
primer coat that's has some gloss to it [or it's an oil based primer].
Flat is an absolute no-no in a high humidity room. The loose paint
needs to be scraped off. Since I tend to dislike doing things over and
over and cleaning flat paint can be a waste unless there's obvious
residue (and I don't know enough about your bath), one way is to do the
walls/ceiling with a better primer/sealer. At the least you should seal
the ceiling and any other spots.
The blotchiness could be one of several things... the primer/sealer
didn't work, moisture is coming from the backside of the drywall, you
have hard water, there's soap residue.
I would go with an oil-based primer/sealer, followed by 2 finish coats
of latex that has a gloss (satin, semi-gloss, gloss... it's your
choice). Avoid using the shower the day you paint.