The bug

Whenever the wither's side heads are facing at their target at almost 360 degrees while the main head isn't looking at the player, the heads aren't rotated symetrically

One of the side heads is very close to the wither's main head while the other side head is further away from the main head as seen in the screenshot below.

To reproduce

1. Run the following command:

    /summon wither ~ ~ ~ {Attributes:[{Name:generic.follow_range,Base:-1.0},{Name:generic.movement_speed,Base:-1.0}]} 

   • That command will keep the wither in place so that it doesn't move to often.
2. Give yourself a shield and switch to survival mode and try to get behind the wither as it may randomly look around.
   → You'll see that the wither's side heads are rotated asymmetrically