I made a map, then I went into the Nether, and opened it. This made a map of the area in the Nether around me (as expected). I then left the nether, and expanded my map I just created (surrounded it with 8 pieces of paper in a crafting bench) in the overworld. When I pulled the map back out, it was completely blank.
I tried to fix it by switching it out and back in again, and by re-entering the nether, but it made no difference.