[32-40win]

Separation is usually from case stretching or even perhaps old improperly made cases, and also pushing the shoulder back a bit far with the sizing die, and from losing springback in the material. If the material won't release from the chamber wall, the base web area stretches because there is nowhere else that can move, and the primer backs out before the base moves, then the base goes back and pushes the primer back in. It may chamber fine with a sized case, but not release as it has no springback left. Which also makes a sticky lift. And it doesn't take overpressure to do that. Although, you likely possibly ought to have been seeing split necks before that, or when sizing them. Brass ages and loses springback also, even new unfired brass, usually takes more like 50 yrs plus though, as far as I know.. It would seem that it shouldn't happen on 2004 brass, but, it is within the realm of possibility, on an odd mixture of material, or improper heating sequences, when the brass was made. If it did that with new once fired cases, or factory loads, then it is either the gun or the powder. Check the old cases you have with a paper clip, should be able to feel stretch/separation grooves above the base before you see them as a ring.
Stranger things have happened than getting a fast lot of powder, it happens, been there, done that. The manuals are done with a lot of rounds fired and tested these days, and they publish it with a mean average pressure rating, so, it can be a combination of things that occur. But it isn't often that it is the powder or the manual being the issue. Manuals can have misprints, too. They usually catch those PDQ.
I'm looking at data for my 7-08 today, old manual shows 49.0 gr as a max load, new one says 46, been shooting that gun with 48gr for 30 yrs, same case, powder and bullet, but with a magnum primer, not a standard one as per the manual. And another bullet mfgr which usually shows smaller charges due to being monometal bullets, shows the 49gr charge. I understand your skepticism.