Ender Dragon healed completely after I died and/or logged back in

I was a third of the way to killing the ender dragon when I did the log out/log back in glitch to not take fall damage. Not long after logging in I got killed by the dragon, after that when I returned to the end, the dragon had full health. 

I am very irritated by this bug, as I spent 20 minutes hitting the dragon in Survival.

Please fix it ASAP!!!!