The problem appears to be the priming punch. I inspected it under a good magnifying glass and it has some machining marks that seem to grab the primers. I bought a bunch of these in a lot years ago and have slowly used them up as I found unexpected berdan primed brass.
I picked up some new ones yesterday and they were nicely polished and clean. This solved the problem.
10mm and 357sig, the best things to come along since the 38 super!