Can’t poison wither skeleton

Ok so I was messing around with piglins in my creative Mincraft world and I wanted to know how many piglins were needed to kill the wither skeleton. Then for no obvious reason, I tried giving effects to mobs. I tried poisoning a wither skeleton and it didn’t work. 

How to reproduce:

1. /gamemode 1 
2. /summon wither_skeleton
3. /effect @e [type=wither_skeleton] poison 30 1

Observed results: The wither skeleton doesn’t get poisoned.

Expected results: The wither skeleton does get poisoned.