disappearing wall of maps

I have a wall of maps. Yesterday when I returned from adventuring, I was shocked to see my wall empty: no maps, no frames. I quickly used task manager to kill the process in hopes of having to avoid loading a backup.
But the program seemed to save and exit normally, and when I re-loaded the game my wall was back to normal (and I was standing in front of it where I had just been).
This is my second post on strange behavior with that wall of maps.