I don't have a solution for you, sorry, but what you're experiencing is probably because it's a planar cut on a rounded element. The cut has some depth to it (modifying the view depth requires a value greater than 0'-0"), so Revit doesn't acknowledge an exact intersection between the cut and the edges. It's similar as to why a live section that's ever so slightly skewed from a grid line will not show the grid line in the section view. In the few instances where I've run into this, I've never had to be exactly in-line with the edge, so the "close-enough" rule worked for me. Sad, I know.
