The sheet is 11x17". All data are in a file geodatabase. The data is rotated 90 degrees. I have exported that single polygon to a different feature class with the same results. The results are also the same if I import that single polygon feature class into a fresh map document.
I'm completely befuddled. I plan to try a shapefile, but in the meantime I wonder if anyone else has any ideas as to why this is happening and what can be done.
Converting the shape to a shapefile fixed the situation, but doesn't satisfy my need to know what happened and why.