What's causing this puncture?
It looks like a glass cut to me.

It is common for glass to lurk in the tire case, so this is unusual.

Take the tire and tube off completely and inspect the inside of the rim and the tire carefully. If you can't see anything, run your fingers around the rim and the tire.

If you still haven't found anything, take the rim tape off and check it and the rim again.

If you still haven't found anything, reassemble, sigh deeply, and hope for the best.