The bug: 

When the wither's foot is inside of any block that has a hitbox or a boat, the wither's skull would explode immediately. If there's piston pushing blocks at its foot, the skull will explode at its foot position.

How to reproduce:

method 1

1. build a contraption like this.
   
2. summon in the wither and remove the redstone block before the wither finishes summoning.
   
   
3. the wither's blue skull explodes at its foot and destroys the obsidian
   

method 2

1. build a contraption like this
   
2. summon in a wither using command [to avoid the initial explosion]
   
3. the wither is unable to hit the shulker because its skull explodes immediately.