I was doing things in the dark forest a few days ago and was able to repeat this bug by accident. Small quest spoilers below in case you don't want to read. It turns out that the bug didn't affect all my maps, just certain ones.
It happened again when I was looking for the Holy Grail. Walking into the portal to find the room was fine. But walking back out the same way triggered the bug and removed the map fog that prevents you seeing things that are supposed to be blacked out.
