Okay I figured out what happened. It's not exactly a bug but it's not intended either. At some point some weeks/months ago I was reworking the distance fog system and - as a means of testing it, set the Z key to set the fog to 500 units away from the player, and the X key to some ridiculous distance *away*. It seems I forgot that I had even put that in there.
Whoops.
On the bright side, though, you can fix it by just pressing X - though it ruins the moody fog effect. Thank you for bringing it to my attention.