my money's on detonation. if there was physical interference you'd have done a big end long ago.

its not unusual for detonation to crack a top ring, it overlaps and breaks off a long chunk of headland.

its also normal for the part of a piston under the closed side of the combustion chamber to be clean.

maybe they should have been milled, but you only need .030" static clearance to be safe. you could measure the clearance on the other undamaged pistons if you wanted to be methodical about the cause of the failure.