Withers have this behaviour where their spine angles in accordance with their head. This is used for the charge attack, but also occurs in whenever the wither looks up or down in any direction. This itself isn't an issue, but it is when combined with the fact that withers can swivel their heads around a full rotation, they get some pretty broken poses that I assume are not intentional.

Steps to recreate
1: Spawn a wither
2: Make it look at you by either being above or below it, so it has to look up or down
3: Observe the spine moving in a broken accordance with the head, and the head not consistently facing forward as it should

Here is a screenshot of a wither with its head on backwards looking down. This is not normal.