I don't think its to do with resin, although it could speed up the process, I have seen leather boots do this a lot but when being worn constantly in water i.e. two weeks on a sodden trench in the peeing rain, and a right good dollop of polish nice and thick would bring them back to good, I would guess the leather is pretty naff if its happening already for what we do, my boots haven't seen polish in six months and haven't turned white