I'm not sure if a break on the Die itself would cause what you have shown us. Interesting. It doesn't look completely raised as a Die Crack would be. Maybe Die Deterioration with some kind of Plating issue.
It is not a die crack. What I am seeing is incuse and a die crack is and can only be raised. I strongly suspect there is some zinc rot developing beneath it. As to just what caused it to begin with, I can only guess a plating issue. Whatever it might be, going through the devises and folded over it has to be prior to striking.
This was my thought....it doesn't look like a die crack...it looks like a plating crack...its has a bubbly look to it. Its raised on the outer edges and then cracked in the middle.
Are you talking about the circular line spaced evenly the same distance around the rim? If so it's a worn out die. If not i need a bigger screen ghan my phone to see it. Lol