The bug
The wither is not agressive towards you since the snapshot 16w35a.
It does fire at you, but does not follow you till you hit it.
Linked issues
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Comments 5
Confirmed for 1.13.1 partly, if you hit him and try to run away from it, it will stay following you even with speed two. if there are alot of mobs around you, he will attack them and then you can escape him. If you go back to him after, he will not follow you, only fire towards you, but if you hit him again he will follow you.
Duplicate of MC-29274.
Can't reproduce.