Poison and regen potions don't work on undead

Skeletons and zombies don't get poisoned when I poison them. Haven't checked the regen potions since a few snapshots ago though.

Poison arrows do work on creepers, spiders, villagers, and wolves, and probably other mobs that I haven't bothered to shoot yet. Or mobs with low health that die instantly when I shoot them.

Also, could you check that the arrows of harming and instant health are working as intended, because I don't seem to be getting a higher rate of 1 hit kills with these arrows (and the bow has power V and flame).
I don't know how to test whether they are working, because they are instant, they don't give you potion particles.