PPU 6.5X55 brass is considered pretty darn good by reloaders of that round.I can't confirm this, but I've seen it more than once, that PPU was producing the brass for Winchester white box when it was first on the shelf. PPU jumped head first into the commercial production, and had some QC problems with annealing their brass, and some out of spec rims on their 6.5 Swede ammo.
I suspect, out of spec primer pockets. Over pressure will show on the ones that still have the primer in the fired case.
Did you save the fired brass that didn't lose the primer? Got pics of those?