The bug
The Wither's rib cage appears to be shifted half a pixel to the left of the center of its model, making it look asymmetrical.
Linked issues
is duplicated by 1
Attachments
Comments 3

It's actually that the center head is offcenter, the rest of the model is correct.

The heads are lined up perfectly with the center of its hitbox.
[media]
That's really hard to unsee.