Ender dragon has a death beam but is not dead!

I Was messing around on what is the damage of my bow. It has power 2555. It is insta kill for ender dragon but sometimes it bugs out. But thats not the problem, the ender dragon kinda died. It left a death beam on him/her. Great Right?! Well I did it by accident when testing so i don't know how it happened. It has zero health but he is still moving. lel