I've seen this happen, too (2.5dev 12/12/23), both the mansion map and the huge parkhouse in some other city map.
Is there a way to unpack the maps.pk3 file and replace the mansion map with my old copy from 2.4? Or is it some change in the graphics engine and disabling the map is the only solution for now?
And yeah, seeing nothing instead of that broken garbage that obscures everything is preferable, I think. Would it be possible to make something that still lets the player identify walkable floors and maybe walls, even if their graphics fail to render? I remember creeping around the garden of that mansion for a half-dozen turns or so, searching for a door to a stairwell that wasn't there until I figured out that the stairwell only starts one floor down...