Two thoughts I'll toss out there for your consideration, and not saying I'm right.

1) Could the bubbles be 'fisheyes'? This is caused by silicone (usually) if the thing has had a lot of Armor-All or something used on it (maybe on the top?). That stuff will stick darn near forever and is very hard to get rid of. It will make little round eyes in the paint sort of like bubbles, but they aren't bubbles.

2) If you are getting bubbles, is your foam roller different than what you used prior? If you get bubbles right after rolling and they don't go away, maybe give the Brightside 'tip' treatment a shot. Go over it with a foam brush and whack the tops off the bubbles to help them lie flat.

Sounds like you've got some great projects there. I love old trucks and Jeeps. Hope you can get this figured out.