Any time there's a pinhole in the body and you paint over it you will shortly see a bubble. I had a panel once it drove me nuts like that. When I ripped the upholstery out behind it there was a rust spot the size of a silver dollar where the bubble was. Did you metal prep the metal ? Metal prep with zinc is great for treating welded seams. I did a video in 2019 , definitely not an Academy Award winner. I took a old fender that was pinholed out and loaded with pits when I was done I threw it behind the shop ,last winter it was buried under two feet of snow. It looks like it did the day painted it and I will post a picture of it. Nothing is better than new metal but sometimes it's not feasible. Heres the video and later on I'll take a picture of the fender
https://www.youtube.com/watch?v=Y-cNT6gjO78&t=207s