I've always assumed that the Hornady GPB was made by Hornady themselves, but it's possible they contract it to REAL?
I've used a fair amount of them in my .54 Renegade, back when they made them in .54 (I still have 4 or 5 containers of them rat-holed for myself), and have definitely found a few that had moved off the powder charge. It can happen for sure. The lead is pretty soft. They will start fairly hard, but as you push them down the bore they move easier and easier as they engrave. In my TC they were moving pretty easily by the time I seated them.
The times I caught them off the powder was always after being carried on an atv though. I carried the rifle on the front rack of the atv, in those "U" type rifle carrier mounts you can clamp to the rack tubing. I got into the habit of checking them before every hunt, I never found one not seated after I carried the rifle while actually hunting.
So, I would get to where I wanted to hunt with the atv, check that the bullet was seated, and then start hunting. When I returned to the atv I would check it again just to see if it had moved, it was always still seated after hunting when I carried the rifle myself.
That was not always the case when I checked them after an atv ride to the next spot I wanted to hunt.
They were very accurate in my shallow groove 1:48 TC. They expand really well and wallop a deer with authority. I just dealt with checking them often, they shot so well I really wanted to use them.