Since biome blending makes visual biome effect transitions configurably abrupt it'd be expected that this would also affect fog, but this is not the case.
Easily reproducible by going to the boundary between two biomes with strongly differing fog colors and moving.
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Can confirm in 21w15a. I've also attached a video.

This is a feature request.
What's the point of closing all biome blend issues as feature requests?
I don't feel like this is a bug, but more of a feature not thought of by mojang, it will be nice to see this tho!