Also had rivet problems
I have had the same issue with rivet stems breaking above the rivet head, starting with a few (3) of the sealed tank rivets, then later with the LP3-4s in the fuselage. I could not definitely determine cause, the rivet, tool and operator technique. It didn't happen during the wing and emp builds, it did happen more with the manual puller, but I had a few with the pneumatic puller. The problem has abated, so I assume that careful attention to make sure the puller is fully seated before pulling can prevent the issue, but the puller and rivet defects may contribute.
I have had the same issue with rivet stems breaking above the rivet head, starting with a few (3) of the sealed tank rivets, then later with the LP3-4s in the fuselage. I could not definitely determine cause, the rivet, tool and operator technique. It didn't happen during the wing and emp builds, it did happen more with the manual puller, but I had a few with the pneumatic puller. The problem has abated, so I assume that careful attention to make sure the puller is fully seated before pulling can prevent the issue, but the puller and rivet defects may contribute.