When loading a world from any previous version, the save data of the new Nether biomes (soul sand valleys, crimson forests, warped forests, basalt deltas) is ignored and the biome that would generate there according to natural world generation is used instead. For all other biomes, the save data is respected.

How to reproduce

This way to reproduce by Darth Cobby uses custom worlds, however this can just as well be reproduced by editing the world save data so that a new Nether biome appears where it shouldn't naturally generate (this edit will be removed when upgrading the world to 20w28a or above), or by using a world from the earlier 1.16 snapshots in which biome distribution was different.

1. Create a new world in 1.16.1 with cheats enabled
2. Download the checkerboard.zip data pack and add it to the world
3. Close and reopen the world
4. Teleport to the dimension 194273:zoo and wait for the surrounding terrain to be generated:

   /execute in 194273:zoo run teleport 0 100 0

5. Close the world and remove the "checkerboard" data pack
6. Download the void.zip data pack and add it to the world
7. Open the world in the latest snapshot/version
8. Travel through the previously generated terrain, noting the biome names displayed on the debug screen
   → Soul sand valleys, crimson forests, warped forests and basalt deltas all report "the_void" as the current biome
   → All other biomes are reported correctly