Re: 6.5 grendel
I'll venture a guess. You have too much powder in the case for that weight bullet. Hodgdon load manual states a max load for the 123 AMAX as 28.7 grains of H335.
Hornady's latest manual lists 26.4 as max with the 120/123 AMAX. I don't have data for the 129, but in such a small case I would think you should back down to 21 grains of H335 and work back up. Also another choice would be to go to a slower powder like varget or CFE223.
I will say that I have not experimented with the 129 or H335 so I can't directly answer from experience. Also, I apologize if you did work up to the 26 grain level with the 129, you just didn't indicate that you had.
Learning every day.