Hi OP, this problem isbnot unique to your caliber, but many once-fired brass in different calibres if fired from big chamber guns.
As some mentioned already, RCBS small based die is your solution. It fixed mine on my 308 and to be on the safe side, I bought the same rcbs dies for my 223 semis even though I already have lee fl dies. Never have the problem again like with Lee FL die. You only need to use small base to fix the problem brass. Once it's fire formed in your rifle, you can go back to regular neck sizing and fl sizing
The OP did buy the brass from someone else. I think you are onto something. Maybe it's not the OPs dies or rifle, but the brass that is causing all the issues.
I am assuming these cases are the ONLY ones the OP has? No cases that were originally fired in your gun to compare to?