Ender Dragon regenerated health after all crystals are broken

What I expected to happen was...:
I expected the dragon to lose health and not regain it.

What actually happened was...:
there was visual glitch of the beams from the cyrstals to the dragon even though they were broken and the when that happened the dragon regained health.

Steps to Reproduce:
1. I killed the dragon the first time.
2. then I went back to my spawn to restock
3. then respawned the dragon. 
4. I broke all the crystals.                                                                                                          
5. then started killing the dragon.                                                                                                                             
6. the visual glitches happened and the dragon regained health.                                                             
7. opened my game to lan to see if all the crystals were broken (they were).                                       
8. went back to survival. closed the game to get out of lan.                                                                         
9. opened the world back up and the regeneration stopped. but visual glitches persisted.          
10. I killed the dragon