It's an outside chance, but if you use a thumb slug there could be a hairline crack or a miniscule rough edge that isn't even noticeable. But your thumb on release is finding it just fine. A friend of mine tore up her thumb in the same spot one night, and when we checked the thumb hole, there was a crack about 1/16th of an inch from the bevel edge. Our pro shop guy said he had never seen a slug crack horizontally like that before. It's worth checking.
Deb