The Wither is able to move and attacks mobs as soon as it is spawned in, even before they stop being invincible. When it stops being invincible, they explode and continue to act normally. While the glitch is occurring, the Wither acts as if it is low on health, and won't fly. This is surely a glitch, and it can be deadly to any survival player.

How to reproduce:
Spawn Wither using the normal Soul Sand and Wither Skull formula