I have not had the problem with either flint nor caplocks that I have. Sorry, cannot suggest anything. My caplocks have patent breeches which protects the wood and the pan, fence and frizzen do that on the flinters.

Perhaps it is a gap problem as Coin notes.
The flinters get a big of fouling on the wood ahead of the lock, but it is not burning the wood.

Poor leaking threads I suppose or holes bored by Pyrodex phony powder fouling can cause burning the wood. I've seen that happen on 2 guns so far, a custom rifle and a TC, where the breech plug was actually perforated, ie: achannel eaten through to the outside. The custom rifle's hole went out the bottom of the breech block, which pressure-burnt a groove and hole into the stock beneath the barrel in just a few shotws. The owner was lucky he spotted it and the gun could be repaired.