I add fog underwater. When I use StandardMaterial it works well, but not with MixMaterial.
If I put my camera underwater, I get the fog, but if I go back above the ground, the fog is stay on the ground, it seems to have disappeared from the rest of the environment because we see the Sky. Only the terrain has a part of the fog which is very strange.
On this PG, go with the camera underwater (the fog appears) then go up above the ground (The fog is on the ground but shouldn’t)