Wither Skeletons seems to have issues pathfinding when it comes to staircases and slabs, sometimes they figure it out but the other 99% of the time they just stand there and don't know what to do.

In the video example I've shown the Wither Skeleton pathfinding VS the Husk/Zombie pathfinding. This has been the case for a while, I'm not sure what's causing the problem but I hope this is an easy fix!

(Sorry for such low quality video, I had to make it fit the 10MB limit.)