Wither Refuses to die

My friend has a realm, and one of the guys on there somehow "Accidentally" spawned a Wither. He lost all his stuff, and him and a couple other guys got on. They "Killed" the wither. But it refused to die. The Wither did the dying animation, but continued attacking. As the animation stopped, it didnt drop a star, and turned invisible, and continued attacking, Faster and more often. He said it shot about 10 skulls at a time. And 2 - 4 shots Full Enchanted Diamond armor.
The area where the wither is at is now called "The deathlands."