According to a user on the MC Forum:
Also, just as a heads up, I gave one skeleton warrior a sword with mending on it, and all vanilla mob kills result in no exp gain (sword isn't repairing either, so the mending enchant is nixing exp gain while doing nothing). Mod mobs seem to still give exp however.

Turns out they were never causing XP drops in the first place. As for mending, it will work now, for Skeleton Warriors only.