Creeper & Spider Item Drop Bug

Either by the player or by a mob grinder, spiders and creepers are not dropping any loot. I have ran my mob grinder for a few hours, gathering stacks of drops from Zombies, Skeletons, etc. but no gunpowder or string is there. I have checked, and they are still spawning and being killed like the other mobs, just no drops. Using latest snapshot.