If you move around the mouse on an observer point, the fog gets thicker and thinner depending on where you move the mouse.

[SomethingDeveloper]

If you're on an observer point in spectator mode and move the mouse around, the fog gets thicker and thinner, as if you are facing away from it or facing it. It's very noticeable on dustbowl.

[chloecormier]

Could be related to fogblend? Maybe the method it uses for calculating the player's view direction is independent of your view being locked to the observer point?

[SilentFox3]

Adding onto this, this also applies while you're in thirdperson and use thirdperson_mayamode or similar to rotate your player without moving the camera's location on-screen. Perhaps more generally, fog thickness is calculated based on your player entity's view angle rather than the current camera's?