I built an end portal in creative mode and traveled to the end to fight the Ender Dragon.

"What I was expecting to happen was...:"
I was expecting to defeat the Dragon like usual then take the exit portal home.

What actually happened was...:
After destroying all of the Ender Crystals and taking out 1/3 of the Dragon's health it vanished. When that happened my arrows stopped firing, and my sword couldn't inflict damage to the endermen. I tried to escape by jumping into the void and took no void damage even at y-500. Then I tried the /kill command, but again no damage, so my character is stuck in the end.

Steps to Reproduce:
1: Build a portal to the end in creative then travel to the end.
2: Destroy all of the Ender Crystals.
3: Fight the Ender Dragon until it flies away and disappears.