Someone, maybe one of the lockmaker's sites, has high speed photography showing the frizzen rebound striking the flint.
Someone else on this site suggested, I think, extending the top part of your leather to protect the top of the flint during rebound.
Bevel up or bevel down? I'm about the last guy here to know about flint lock geometry, but I do know that different design of locks perform differently with bevel up vs down.
The very last thing resort, IMHO, would be to have someone work on the lock.