The bug
The nether fog behaves rarely when you're located deeper than y=0. It should display a gradient instead of going from 'full-vision' to 'no-vision' (see video attached).
How to reproduce
Go into spectator or creative Mode
Run '/execute in minecraft:the_nether run tp @p 0 0 0'
If you look above, you can only see a circle of bedrock and there isn't any gradient or anything like that
Linked issues
relates to
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Fixed in 21w15a