Newly found end cities despawn chunk by chunk. Previously found end cities before update are unaffected. Also not sure what is going on with the "black" lines in the end islands that load and unload.
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I took a stroll over your world and noticed the black lines flashing in the end, these are just chunks reloading which is unusual to happen so often, but not alarming.
I have yet not noticed end cities to despawn though. Please comment if the issue still occurs.
Cleaning up old tickets: This ticket had been set to 'Awaiting Response', but has not received a response from the reporter (~3 months+) so is being closed as Incomplete. If you feel this is still a valid issue then please comment, or create a new ticket following the Issue Guidelines which includes steps to reproduce the problem.
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Can you upload the world affected to the issue? If the file is too big to be attached you can upload it elsewhere and link here.