end gateways not generating after dragon fight, like right on the tin, they just don't generate, weird...
all you have to do to reproduce is to go to the end in 1.16.4 and kill the dragon more than once, then delete the end file in the world save, load it up in the new version and kill the dragon, and (*cricket noises*)nothing happens
I have attached some files I hope can help
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Did you defeat the dragon on a regular or custom world type? If custom, check on which biome the main end island is.
it was a Superflat world, Redstone ready and also to reproduce you don't have to delete the end, but its what I did the first time, so defeat the dragon in 1.16.4 and then update and repeat, no gateway will apear

Please provide a screenshot with you debug screen enabled (F3)
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Well I don't know about the bug but you can always use a honey block or slime block flying machine to transport yourself to the end islands (even before you kill the dragon).