What does it measure, I.D, thickness, width? Might help you narrow it down. Anything funky going on with the boattails of the TMK's? Any wild, unexplained fliers that would indicate bullet failure?
There is a possibility: when the bullet was swaged in the point up die, the seating punch that forms the B.T. could have pressed a ring/crease into the base as the bullet point was being formed. Same thing could have happened when the core was seated, to much pressure there and the jacket will crack right at the base. If it was a bad punch, the whole box would show this. Or an untrimmed jacket got into the production run, everything got squeezed and the over flow of jacket material went to the base. A fat bullet with a damaged B.T. You would have noticed when seating. Or the jacket was cracked during core seating.
A lot of bullet dies are designed to have a .0002-.0004" fat base. Better gas sealing. If they get to fat, pressure builds up pretty fast or they blow up right out of the muzzle.
This is all assuming it is jacket material, and a somewhat wild assed guess on my part. How CFE 223 could be involved, I have no idea.