Wither sometimes stop in place while fighting the warden

I don't know how to repeat. But I accidentally recorded a video. This bug happened a lot when I had the Wither fight warden in the normal world and I need you to fix it 

Steps to Reproduce:
Create a normal world and summon wither and warden and watch the battle.

Observed Results:
Wither sometimes stop in place while fighting the warden 

Expected Results:
Wither should be able to fly freely and not stand still while fighting.