What I expected to happen was...:
Ender Dragon to die normally.
What actually happened was...:
Ender Dragon emitted light as if it was dying but continued to fly around in a sporadically.

Steps to Reproduce:
1. Locate/Summon Ender Dragon
2. Kill using sharpness 20000 or anything that will 1 shot it (Might not need to be one shotted though)
*Dragon will NOT die if it is a summoned Dragon. Natural Dragon will die normally with the exception of extra health after it's health bar is depleted.