Do you have another seating die? I'd try a different seating die to see what it does. How many firings on the brass? It's possible that over repeat firings that the necks have grown thicker as brass has migrated from the shoulder. Your not using any kind of expander I assume so the outside of the necks is the only thing you are affecting with the bushing die. If the necks have grown thicker there is no longer 1thou neck tension. Maybe measure your new brass with seated bullet and old brass with seated bullet and see if the necks are any thicker. It does seem the bullets are definitely deformed more on the old brass than the new. Perhaps you got a batch of really soft bullets and They are just showing something in your brass that has been slowly adding up for a while.