If you stack up snow layers to make it a full block and stand on it, the fog disappears suddenly.

What I mean by a full block: (The one in the center)

When standing on a full block of snow

When standing not on a full block of snow

EDIT:
Mod version: 1.1.1
Forge

How odd. At a wild guess, I'd assume there's something with how snow layers actually have a smaller hitbox than what is rendered (similar to soul sand in 1.16). Otherwise I'm not sure why it would be happening specifically with snow[layers=8]

Can confirm this happens in the 1.16.3 build, too.

I can try to do some testing and see if it happens on blocks similar to the hitbox that the layer 8 snow block has, not unless you already tested that

Alright, so I've identified the problem:

• When the player is standing on a full snow layer (or soul sand), their position is actually reported as on the block below - due to the fact these blocks have a less than full hitbox.
• Both of these blocks also block light, and namely, getting the light level inside these blocks is reported as 0.
• Primal Winter uses sky light level to reduce the fog effect when you're indoors. This is intentional as otherwise caves and other structures would look very odd with a white fog effect.

Luckily, I should be able to fix this by checking the light level at the player's eye height, not the feet position.

It works, nice!