In underground missions, in caves, the walls or sometimes open spaces turn black. My captain and bridge officers disappear and I can't see what's in from of me.
Sometimes it's a wall and I don't know that I've stopped moving until a sound occurs or sometimes it's the enemy and I walk into an ambush. The black section in front of me is marked by a straight border not shaped by the terrain. This only happens in caves. Not during boarding parties or in buildings.
I had this happen in a recent cave mission, but when I rotated my camera angle around, the map reappeared and stayed. Not really sure what was going on. It happened a total of two times on the map (each time in a new room/area).