The height limit changed to 255 and 0 intsted of 312 and -64
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Comments 3
This feature, "Works As Intended", as stated in the 21w15a changelog. Quote - "World height has been switched back to the 1.16 levels".
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We're tracking this issue in MC-223144, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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The world generation has been switched to 1.16 style so you won't be able to build higher than y = 255.