When I throw a splash potion of harming I expect it to harm living mobs and that is about it. What happens is the mob is also knocked back even zombies but the undead mobs still don't take damage even though it looks like it. A splash potion of healing kills undead mobs as expected but doesn't cause knock back like the potion of harming.

I would expect that no knock back occur with a splash potion of harming or that knock back from the potion of harming/healing only knocks back the mob if it actually hurts it.