I am 100% certain that the mist is spreading while I’m not playing the game.
I’ve saved and restarted several times without issue. This didn’t seem to happen until after the first Lair appeared on the map. When I saved my game there were only 4 small patches; I just continued and now there are 3 absolutely massive patches much farther inland. Only my first base was mist-covered, but now ALL 4 of my bases are covered.
I reported the issue via F12, and will start keeping screenshots to compare.