While in The End, I tried to go through the portal to the Outer Islands, but was teleported back to the initial End spawn point. I then went back to the exit portal, and was given "The City at The End of The Game" advancement.
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Yellow01
migrated
I suspect this is related to the end city problem. I don't think I have seen any gates either. Maybe because nothing exists within range it defaults back to the spawn platform. I can explore my end dimension later and see if there are any gates generating. This should probably be linked to MC-140728.
migrated
you praobably were using an old world preset
While the first half of the bug is already reported as MC-138069, I don't think I've seen any reports saying they got the "city at the end of the game" advancement, though it might be caused by MC-140231