Steps to Reproduce:
1) Open 1.19.4 and enter the end
2) Run /fill -50 255 -50 50 255 50 obsidian
3) Update to 1.20.1
5) Kill the dragon
6) Notice that the dragon's end portal has spawned at the very top of the world placed with the fill command.
Notes:
The old 1.19.4 end portal can not be used to respawn the dragon but the new one can.
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Comments 3
It's related but I don't think it duplicates. That bug is about the portal being cut off from the bottom. This bug is to do with the game ignoring the portal generated in 1.19.4 and redoes the placing of the portal at the highest block of 0 0.
The other issue doesn't really explicitly state being cut off at the the bottom, just incomplete in general (one of the screenshots even shows the same result here, with the portal clipped at the top of the world). The primary example and provided world just seem to show the bottom of the world.
Also, whether or not the old portal is present or not when the new one is generated doesn't seem to factor into the result. The issue is generally just the portal being placed incorrectly.
Duplicates MC-93185. The original poster is inactive though, so may be helpful to take over it.