The bug

When cave biomes generate exposed to the surface surrounded by a dry biome (e.g. desert, badlands) or snowy biomes (snowy plains, etc.), some weird issues occur. This is independent of other related but contributing issues, like MC-238506 and MC-239446.

For example, when moving down from a badlands biome to an exposed dripstone caves biome, it suddenly begins to rain. This does not make any sense, as the rain would logically have to come from above, and the biome above does not provide rain.

This was commented before here but I couldn't find a report (edit: I found MC-241542, which covers the vertical part of this issue).

To reproduce

1. Create a world with the following seed and go to the following coordinates:

   Seed: -4508336170667350008
   Coordinates: /execute in minecraft:overworld run tp @s -158.82 98.60 51.38 67.35 -29.10
   

2. Notice that you are in a badlands biome.
3. /weather rain
4. Wait a few seconds.
5. Notice that it is not raining.
6. Move down, and notice that it starts to rain, which is nonsensical.

Potential fix

This could be fixed by changing the cave biomes precipitation to be NONE (which I do believe makes sense, why would it rain underground?), but that might be undesirable for other reasons. A more difficult fix would involve checking the topmost biome or the highest biome at a certain range from the player instead of checking the biome at the position of the player.