The bug

Cauldrons are supposed to be filled with water while it's raining, and powder snow while it is snowing. Frozen oceans have a unique temperature gradient that makes it snow in certain places and rain in others. However, whenever a cauldron is placed in this biome, it is filled with powder snow regardless of which local region it is in, even if it is visually raining. Ice and snow are formed as normal (i.e. following the boundaries of the local temperature values).

How to reproduce

1. Find a frozen ocean, or create a superflat world with just frozen ocean
2. Use /fill command to create a large layer of cauldrons under open sky
3. Change weather to raining/snowing
4. Observe that some cauldrons are filled with powder snow even if it is raining
5. Replace the platform with solid blocks, and see how snow layers are formed normally

This bug does not exist for the deep frozen ocean biome.

Code analysis

Code analysis can be found in this comment.