I also shoot a 257 but this happens with many other cartridges as well, belted or not.. Your problem is that even backed off a full turn, the case body is still being resized, at least partly, by the FL sizing die. This happens when you have a chamber a bit on the larger side as the FL die is usually built to SAMMI min measurements to ensure a fit in ALL 257 Chambers. As you push the round into the die it squeezes down the shoulder body junction, which creates the effect of pushing the shoulder forward and lengthening the overall cartridge length. The shoulder does not get pushed back into place as the die is turned out 1 turn. If you FL size the brass they will fit fine again, and likely won't need trimming except every third or fourth shot.
That said, FL sizing you are working your brass a lot more than it needs to be because every shot the brass will have to expand to fill the full chamber. Might as well just leave it where it is once formed.
The neck only die will solve this problem.
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