I looked over previous bugs and issue reports and this seems to have been an issue in the past, but has resurfaced in my game. I spawned it at spawn, because I like to have a challenging fight, got my butt kicked, but returned with my backup loot and some more potions to defeat it. But, after it flashed death red and turned sideways, the heads it shot and land destruction attacks continued, to no end. It's still stuck this way on my friend's realm. Upon further returns to the site, the wither is visible, but completely invincible.