Did restarting game solve the problem? :whistling:
No. It first appeared in my saved game, and new game too.
I believe it might be tied to how much memory/disk space is available and perhaps the size of the save game. If your memory or disk space are low and/or your save game is large, it will likely occur after several minutes of play. Exteriors are more prone to this. I'm guessing it's because more cells are loaded at a time with each cell change.I think It's best to nip it in the bud. If you see it starting to happen, reload your game. I've tried the Update Map feature in Wrye Mash (with the MCP fix running), but that did not completely restore things (plus it leaves grid lines all over your World view).
Despite starting a new game, the same problem appears. Since the size of the save game was nearly 4MB, this problem didn't occured before.
My hard space still remain 1/4... it doesn't look like the cause.