Yes, I get it occasionally too. This bug has been reported many times to MM over the past 6 months. If you have a save game, go ahead and send it to them.
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No bastard ever won a war by dying for his country. He won it by making the other poor dumb bastard die for his country.
-General George S. Patton
An alternate way to force a refresh of the corner is to "alt-tab" another window to the front, and drag it over the spot, the move it away. Windows will ask SE4 to redraw the affected segment, & it should return to normal.