Withers will not die while in the void; instead, their death animation continues indefinitely producing some odd graphical effects.
Steps to Reproduce:
1. Dig out a 3x3 hole that leads directly to the void
2. Spawn or sumon a wither in the middle of the hole
3. Wait until the wither's health is drained from taking damage while in the void
When the wither falls through to the void, it begins to slowly take damage. Eventually, when the wither's health is fully drained, it begins it's death animation; however, the death animation never ends and the wither never dies.
The wither should die as it normally would, and carry out its death animation for a short amount of time.
This bug does not occur when the wither is summoned in the void using the summon command; the wither must have time to charge and move on to its attack stage for the issue to occur.