I buckled some shoulders when I was using RCBS dies for my 303 Epps brass, once. What the issue was me trying to put a roll crimp and seat at the same time, with slightly inconsistent brass. It was all undersized, but varied by a few thou from case to case. Whereas I set the die to crimp on a shorter case, those that were a few thou longer were buckled.
I was able to resize them and reload after I figured out my error. I seat with the RCBS die and crimp with the Lee Factory Crimp die now.
I note that you were full length sizing, it is also an issue when neck sizing with Lee dies that an improper adjustment of the die and press will result in a buckled shoulder.
I can't post the pic of the instructions, but in the link below open the Collet die instructions and see the "buckled shoulder" section, it may be applicable in this case.
https://leeprecision.com/instructions.html