Mob can not drop blocks (not all)! If you select item to mob's drop it is works. If you select block (VINE and NETHER_WART) to mob's drop it is not works. This bug in MCreator not in forge.
P.s. I don't use event (when mob died)
Issue comments
I can confirm this, but it seems that this is a Minecraft Forge level bug as all other blocks seem to work. I can't help to fix this as this is not a bug on our side. I will keep you posted if the Minecraft Forge fixes this, but for now, I am closing this ticket as there is not much we can do at our side.
I have used dirt for a drop and it worked. Could you attach your workspace so I can import it and see what causes the problem you described?