https://github.com/Kitteh6660/MoreCraft/blob/1.12.2/java/kittehmod/morecraft/MobDrop.java#L27-118

Drops should be added by adding new instances of EntityItem to LivingDropsEvent#getDrops(), so that subscribers which are notified later are aware of them. Because of this issue CraftTweaker's drops API currently cannot be used to edit drops MoreCraft adds via the event.

Should be resolved with 3.2 (release) update now.