In my case i created a world some time ago on one of the snapshots with new 1.16 biomes and now when i open the world in 20w29a, the place where the soul sand valley biome has generated, now is identified as nether wastes.
I tried creating a new world with the seed of this world (368836386368683368) and when i go to the coordinates where the soul sand valley has been generated, now there's a different biome - nether wastes. As if the game applies what biome is in what location based on the new world generation algorithm (if it has changed?).
I replicated it like this:
Create a new world in 20w10a.
Go to the Nether at 2, 95, 106 (or any other coordinates)
Update the world to 20w29a
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Indeed. Using early development versions like 20w10a can lead to corruption even in the long run.
Probably a duplicate of MC-194273