This is one for the books and note I am not normally an Xbox One player. This is a world my son-in-law updated from Xbox One to the bedrock version and the dragon had already been killed and respawned on that world before so an exit portal/2 end city portals already existed.

The night before he, my daughter, and his father respawned the dragon and a 2nd exit portal was created on top of the old existing portal - both are active and work. The next day, we set it up so I could join with my Win10 version. He set the world to peaceful so he could show me the world and where everything was - however when I logged in, the dragon was sitting at spawn. Just a ghost as it couldn't be hit and didn't fly. Logging out and logging back in got rid of it.

Since they were working on getting the end achievements (reset on updates to Xbox?), I went along to do mine at the same time. Fight was going normally till I died and coming back into the end the dragon had disappeared. The three remaining crystals were still lit, but there was no health bar or dragon. Respawned it again and this time it immediately disappeared after creation. It was late so everyone logged off. I suspect the double exit portals have something to do with it since until I died and exited the end everything was ok, but have no idea how to deactivate the old one on survival since going creative is out of the question.