The bug

Burning mobs are not extinguished by snow in snowy biomes. However, they are extinguished by snow in cold biomes where the snow starts at a certain height.

This also happens with medium biomes with the snow above y = 256.

How to reproduce

1. Create a Buffet world with biome "Snowy Taiga" (for example, you can choose any snowy biome)
2. Run /weather rain
3. Spawn a zombie when it's day or use

   /summon zombie ~ ~ ~ {Fire:1000s}
   

   → It won't get extinguished by the snow

4. Repeat the process with a "Taiga" Buffet world above y = 140
   → The zombie will get extinguished by the snow