Why does my splash potion of harming not do anything in minecraft?

I brewed several splash potion of harming II's, and I tried to use them on the zombies in my zombie spawner. Nothing happened to them at all, and I just wasted 3 of my potions. Why is this happening?

Anything dead (Zombies Seletons ETC) need instant health to be killed and not harming.

It's a bug. Vey likely you should try them on another mob, if it works, try your zombies again. If potion doesn't not react to zombie, then it a a bug.

Unsure how to fix, Try leaving world and joining back again.

Oh wait, zombies… Dead= harming= heal. Whoops.Use health potions instead.