This was a really weird one that exposes a bug that I'll report.
In Chris' screenshot, it is the wall at the right that is being cut that is the problem. It turns out that there is a window in the wall, completely outside of the section cut range. It is set for 'studwall with siding'. Merely changing it to 'studwall' gets the section to show correctly. Oddly, dragging that wall to the right so that it is not attached to the building also gets the wall to display correctly.
As he said, there were no overlapping elements, no solid element ops, etc. Very surprising to me that a window can affect the display of part of the wall that isn't even near it.
Cheers,
Karl