Kyle, I would full length size the brass after every firing. PMA makes some very nice mandrels, run them through the neck after bushing sizing, and this is standard procedure for many of us. The bushing puts a kink in the neck, the mandrel straightens the neck.
Sounds like it is time to contact K&M.
I have many Lapua cases that have been fired 30+ times with no doughnut, but they are not this caliber. I wonder if you have been annealing too far down into the shoulder? I am guessing, of course. Doughnuts after 4 firings....hummm.....? Kyle, it should not be just you having this problem in this caliber. The question is, did you form the 6.5 PRC from 7 PRC, which would explain the problem?