EnderDragon sometimes doesn't spawn correctly

EnderDragon doesn't spawn correctly sometimes.

What I expected to happen was..the EnderDragon would spawn and the fight would begin.:
Describe what you thought should happen here,the game would load the End and all the mobs would load including the EnderDragon.

What actually happened was,the EnderDragon did not spawn..:
Describe what happened here,i built the end portal and jumped in and everything loaded as usual except the EnderDragon wasn't there so i bulit the clay creeper face and it still didn't spawn.

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
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