R Payne
Here is a paper, while not on wood generally, does show that heat changes the bonding ability of lignin (among other things). It shows a change in dry lignin in some form starting at 125 and at lower temperatures when wet.

I found that with 10 minutes of searching and reading.

It sounds reasonable that if heat can change the properties of lignin when it is extracted from wood, then it can change the properties of lignin within the wood.