According to TVM, using the Chamber Siler lock, the flint size is correct at 3/4 x 7/8.
Be aware that those flint sizes are for the NEWER Siler LH lock, as the recommended flint size for the OLDER Siler LH lock is 3/4” square one, so shorter.
FWIW I recently purchased a new-to-me older LH custom flint rifle and tried to get her to fire using the longer flint and I had similar issues, even though the flint edge is off the frizzen at half cock. I switched to the shorter flint -
that looks TOO short - and installed it bevel down ... and the reliability is 100%!
The flint edge is a good 1/8” off the frizzen face at half cock ... which proves that flintlocks are like women ... ya gotta find out what THEY like! It is quite possible that TVM had an new-old-stock LH Siler on hand ...