After applying Potions - Creeper becomes immune to it

Creepers become immune to Potions.

*What I expected to happen was...:*
Creeper dies when applying Potions of Poison

*What actually happened was...:*
It eventually stops showing harm but still shows potion effect bubbles.
If you then hit it it dies.

*Steps to Reproduce:*
1. Spawn Creeper
2. Get Potion of Poison
3. Shoot Potions of Poison at Creeper