You know, it almost sounds like the die might be slightly buckling the shoulder--which is strange, if it is a full length sizer.
If you are crimping the loaded round, the application of the crimp can also buckle the shoulder.
The only rifle rounds I do crimp are .223 for my AR15's. It can and does send bullets right back into the cases, if I am loading the heavier bullets for that caliber. For the crimp, I use a Lee Factory Crimp Die, adjusted to just barely move the case mouth into a cannelure.
Hiding in plain sight...