Steps to Reproduce:

1. Create a world;
2. Build a portal;
3. Enter the portal.

Observed Results:
When the Nether generates, you can see that the fog smoothly transitions from the overworld sky color to the fog respective to the current Nether biome you're in.

Expected Results:
The fog is instantly updated to the Nether fog when going through a portal.

Screenshots/Videos attached: Yes

Notes: Probably caused by the feature that makes fog smoothly transition between biomes in the Nether.