Expanded/zoomed maps do not show previously-explored area

Expected behavior: when creating a zoomed map, the resulting map will have the original map's explored area already filled in.

Actual behavior: the zoomed map is blank, with only the player's small surrounding bubble filled in.

I believe this is new behavior with 1.5.1 or possibly 1.5.0 because I had a 3/4 map created under 1.4 that I tried to expand last night under 1.5.1 and only now did I notice this behavior.