Placing end crystals where the end portal, for example, is going to generate after you kill the ender dragon causes no end portal to generate.
This seems to only happen with end crystals.

Doesn't occur when portal is generated in end portal frames.

To reproduce:

1. Spawn an ender dragon with end crystals.
2. Place an end crystal where the portal blocks will be after dragon is killed.

3. /kill @e[type=EnderDragon]

4. Destroy the end crystal that you've placed in the portal and see the issue.