I'm attempting a completely naked Skyrim new game right now to see if it's intrinsic. Just the BGS-authored plugins. No loose mesh/texture/script/interface/sound/music files (just renamed folders, not deleted

). no SKSE. No ENBSeries.
Just gotta get through the cart ride...
EDIT: Aaaaand nope. Bare naked vanilla Skyrim is affected too.
EDIT2: Investigations... The v1.8 Update.bsa appears to have a Map.swf in it. I am going to extract the original Maps.swf from the base Interface and toss it into the Interface loose files to see if that corrects it.
EDIT 3: Interesting... The map from the v1.8 Update.bsa is a good 6kb bigger than the base, 70kb vs 64. Going in-game...
EDIT 4: Aaaand still nothing.
This really puts a crimp on navigation.