Biomes on older worlds such as those worlds created in version 1.8.0 are corrupted in 1.21.40.21 preview (and further versions) with no experimental features turned on. For instance, my portions of my swamp biome turned into ocean biome.

Loaded biomes are corrupted once the world was reloaded.

Edit: The older backup of the world (1 week before 1.21.40.21) works well with version 1.21.30.25 preview.

Here's the link to the world (RAR file) in OneDrive: https://1drv.ms/u/s!As6yzl-_8s1DgaIoBgl2I8eYJpv8NQ?e=J3H1eV