Multi-hitbox models/mobs (OBB) not compatible with oldcombatmechanics plugin

When oldcombatmechanics (https://www.spigotmc.org/resources/oldcombatmechanics-disable-1-9-hit-cooldown.19510/) is used in conjuction with models which utilise multiple hitboxes (OBB), they do not register any damage upon being attacked, even though they flash red/react. I have talked with the models creator about this, and he has mentioned that he cannot fix it through purely the model himself.