I think I've experienced this bug before, and it seems to be some kind of problem with the graphics memory when you load a game. I could not find out a pattern behind that problem either. I also observed damaged walls suddenly looking normal in the last few seconds of the game, when you hear the drumroll of victory, but still see the screen.
If the walls only become visible late into the game, a solution might be to damage them just before they get visible; this should reduce the risk of loading a saved game with this graphic bug, and that the damage is still "fresh" when the walls first become visible.
Ingo
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