Not familiar with the die that you are using but when I have seen those type of dents, they have always been a lube issue on the shoulder. That's not to say that is necessarily the cause here but that's been my observation.
For what you posted, this condition only occurs when you attempt to size these cases. Obviously, something is going sideways with your sizing operation. I can only think of three issues that may cause this: (1) lube on the shoulder or accumulated in the die, (2) air compressed and trapped in the die deforming the case, (3) excessive pressure on the cases due to over sizing aggravated by this brand of cases being of a different thickness than the Lapua.
If possible, I would try a standard die or different sizing die on three or five cases.
- Are you getting a lot of resistance when you size these cases? In other words, having to apply significantly more pressure than with the Lapua cases?
- Do you measure fired and sized case head space with a bump gauge and caliper and if so, are you getting several thousandths expansion. In other words, do you have to bump the shoulder?
- As I said, I am not familiar with bushing dies, never used them but is it possible that you are using an inappropriate bushing creating too much stress on the case?