To me it looks like the buffer is hitting the pin just enough to cause the damage.
Slowly close your upper with the BCG in onto the lower. Watch the gap between the upper and the lower where the rear of the carrier makes contact with the buffer. Does the carrier push the buffer off the retainer pin? It's supposed to. In other words when closed, the buffer should not be able to touch the pin.
Are you sure it's happening in all three lowers or is it possible it's only happening with one or two of the lowers, but you have inadvertently switched all three buffers into the one or two lowers that could be causing it? My thought is the buffer retaining pin hole on the 60%'s may be slightly too far to the rear not allowing the carrier to push it off the pin. It wouldn't be off much, just enough to let the buffer strike the pin each time the gun cycles, but not enough to cause the pin damage. However, the finish on the culprit pin(s) should show at least some wear.
Of course I'm just guessing here based on the photos.
ETA: Sometime you will see damage to the buffer face from a burr on the back of the carrier. However, this usually produces scratch marks across the face from scraping when opening and closing the buffer along with the circle damage from cycling and rubbing the burr. Also, since you are saying it happens in all three guns, it would mean all three carriers have a burr - or you are switching the buffers. This could be the problem, but not as likely.