Probably this is an Oblivion bug:
On my local map, areas which I had already fully explored to the last square foot, are suddenly shown as unexplored (plain white). The "missing" parts are all rectangular.
Is there any way to get the missing patches back, or at least prevent the loss of information for the future ?
I have experienced this issue
I have experienced this issue as well. At least I have noticed it in Vivec. I had mapped all the cantons and suddenly *poof* the minimap is black again and I can map it from scratch.
Unfortunately I have no idea what's causing it and I simply ignored it, because I know that I have seen all of Vivec and the map markers are still in status "discovered".