Have you checked neck wall thickness, to see if there's a bigger difference between the cases than just the amount they've been annealed?
With cartridges that are based on something else, Remington has a REALLY bad habit of taking shortcuts, and creating neck walls of the wrong thickness. One of the effects of that is often reduced neck tension. (Due to thin walls, or walls so thick they crack when sized.)
So... you're saying there's a .311-.312" ridge at the base of the bullet, but the bearing surface still measures the proper .308"?
"Such is the strange way that man works -- first he virtually destroys a species and then does everything in his power to restore it."