If you kill the ender dragon and place down the four end crystals before the dragon finishes its death animation, it will drop the xp and open up the exit portal, but the end crystals don't do anything and it will not resummon the dragon. Placing down a 5th end crystal will however.

Steps to Reproduce:

1. Enter the end
2. Kill the dragon

   /damage @n[type=minecraft:ender_dragon] 20000 minecraft:arrow by @p
   

    

3. Place down 4 end crystals on the bedrock of the end portal

Expected:

The dragon will be summoned again.

Observed:

The dragon is not summoned, and the crystals will just sit on the bedrock blocks. If you place a 5th crystal, the dragon is then summoned.

Video:

https://www.dropbox.com/s/zexqvil8l3tsw56/Minecraft_%201.16.5%20-%20Singleplayer.mp4?dl=0