Enderdragon death flashing and does not kill when hit by powerful fireball

When the ender-dragon it hit with a powerful fireball, after some lag for me, it does the death animation, but then the dragon did not die and kept the 'light beam' effect on it. As well if the dragon does die it flash multable times with 1-2 secs.
I did a video on me tring to recreate this glicth/bug/whatever: https://youtu.be/acjHLIHg9-I