"These callbacks must not call exit()" does not mean "If these callbacks call exit(), special interesting things will happen". It just means "don't do it, or you're on your own". It's possible that a different POSIX-compliant system will do something else, like an infinite loop. Since you didn't follow the rules, you can't count on what will happen.
(I would assume that few if any systems would get into an infinite loop, though. It's trivial to avoid that, and I can't imagine it being a useful result.)