Dragon Does not respawn when End crystals are placed

What I expected to happen was...:
Once the End Crystals were evenly placed on the exit portal the End Dragon should start to respawn.

What actually happened was...:
Nothing, the End Crystals just sit on the Exit portal and do not start the Dragon respawn process.

Steps to Reproduce:
Simply place the 4 End Crystals around the Exit Portal in the END.