It is no BS...vinegar does etch metal surfaces. We had copper gutters installed about a dozen years ago. My wife loved the color of the new copper, but, once the patina process set in...she hated it. So, I ended up painting that expensive guttering. Wiped it down with vinegar, primed and painted and it looks the same now as it did 4 years ago. Btw, apple cider vinegar is good for what ails ya.
I do not believe that adhesion of the paint to the surface has anything to do with the problem described by the OP. IMO, the possibilities have already been suggested....
a) the paint is not completely dry
the top coat is being applied too heavily
I see the problem as one of the two....or, both.
Good luck.