In worlds created in the "floating islands" mode, where the biome is selected as one of the biomes where snowfall begins above a certain height, when updating these worlds to version 1.21.2 or 1.21.3, the snowfall boundary is at about 128 blocks lower than it should be.

Steps to Reproduce:
1) Create a world with the “floating islands” generation type, choosing as a biome one of the biomes where snow falls only starting from a certain height (for example, wooded_mountains aka windswept_forest)
2) Open this world in version 1.21.2 or 1.21.3
3) Make it rain

Observed Results:

It's snowing at ground level (in some biomes like taiga variants the snow starts a little higher but lower than expected)

Expected Results:

It's raining at ground level, the snow lines are as shown on the wiki

Screenshots/Videos:

The screenshots show that in version 1.16.5 it is raining at the surface level, and in version 1.21.3 it is snowing in the same location in the same world.
The approximate lower limit of snowfall in the affected world is also indicated.

Notes:

Tests were carried out for worlds created in versions 1.13 (where it was first discovered) and 1.16.5, but most likely the same bug exists for any versions where the "floating islands" generation type exists.
Despite its specificity, the bug is quite destructive (snow and freezing water significantly affect the landscape and buildings), which has a very negative impact on the experience of those few players who play in the affected biomes.