I wish the whole "2k" description would leave the paint world, I have to assume "2K" means Urethane, but being every product that you put a hardener in including epoxy are a 2K I am not sure what was used here.
But anyway, was the primer sanded wet? How long did you wait after sanding before base coat was sprayed?
The lack of adhesion is between the primer and base is what we are talking about so something to do with how it was sanded and how the base was applied is the issue.
First off, 600 isn't too fine if everything is done properly, heck, you don't even use 600 on a blend panel, 1000 is often used, or even a gray scuff pad. I am thinking either the primer was wet sanded and it wasn't throughly dry that could be an issue. It is unlikely being it's such a big job, unless these panels that are peeling were the last ones sanded right before it was painted. Is it peeling everywhere? Or is it just these panels, and are these the last panels sanded?
Next possibility, were these the last panels sanded and the urethane primer "healed" before it was painted? Some urethane primers can do this more than others. One primer I use has something crazy like a 4 hour window after sanding to paint, it "heals" that fast. So if it was sanded and sat for a number of days this could be the problem.
That and of course the metal temp was too hot and the basecoat flashed the second it was hit and didn't bite the primer.