This right here. Issue didn’t start right away. I’ll measure the case above the rim and see how it compares to some virgin brass. My full length rcbs sizing for is camming over so I don’t think I can size any farther down with that die.
You made he have to think hard and go over some of the problems I've had over the years.
I've had it happen twice, once on some ammo I was only neck sizing, and once when it turned out I had a FL RS die that was out of speck.
In the case of the first I just had to adjust the die a bit to get the proper shoulder bump and in the case of the second, I sent some resized brass back to Hornady with the die and they gave me a new die.
If you can find a small base die set for reasonable money that might just solve all of your issues.