After opening world last played in 1.12.2 (or older version of the game) in 21w39a snapshot can cause random chunks to generate from scratch.
Bug affects worlds opened in new versions of game (1.13-1.17) with parts of terrain visited/loaded last time in 1.12.2 or before.
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is duplicated by 1
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If you generate a new 1.12.2 world and inmediately upgrade it to 1.18, almost every chunk gets regenerated, I've recorded a video and here you have the link to watch it: https://www.youtube.com/watch?v=o_i-wGnuZ3s
It seems like the underlying cause of this issue may be MC-238056. Could be a duplicate but could also be helpful
This may be due to using an old world in a new version, and that’s what I Expect is happening, although I’m not exactly a Mojang staff member or anything.