When I came to the end, there was no Ender dragon, no portal. I checked in another world, he's not there either.
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So, in order to say really that this is a bug, nothing of those should apply:
This is a multiplayer world
Another player has played on this world with the same device
This is a realm world
Ender Dragon has been defeated on this world
If none of those apply, then its a bug. Reply to me if you want,

This is happening to me too. I tried to Reset The End, and no Ender Dragon is spawning. The ender crystals are there, but no dragon.