The exit portal at the end stops working and the dragon cannot be revived

I'm playing with my friends on my multiplayer world where we had already killed the dragon 5 times. Another friend came into the world and the first thing he wanted to do was revive the dragon. He revived and kill him, but when the dragon's death animation ended, the return portal did not appear. Our friend told us about it, so another one went with more end crystals to revive the dragon again to be able to solve the problem, but when he placed the crystals, nothing happened. I made a copy of the world so I could investigate the problem in creative, and indeed, there was no way to revive the dragon, or get out of the End without dying.