Mobs, when killed by Iron Golems, don't give their drops.

I recently built a fully automatic witch farm, but it didn't drop the witches far down enough to kill them, so I added an iron golem above the hoppers. All worked great, until after 4 hours of max-efficient afk-ing I still have none of the drops (Redstone, glowstone, sticks, spider eyes, gunpowder, sugar, bottles). My theory is that there is a bug that makes it so none of the witches killed by iron golems actually dropped anything, because it was the iron golem that killed it. I tried killing witches manually, and it did work correctly then.