This is another great example of a corridor - in this case for a long ditch that is defined against an alignment that is not Curvilinear in nature - i.e. it is a series of break points. In this case the customer created a Template at or very close to a break point on the HAL and the Road Surface was created on the outside of the break angle but not on the inside of the break angle. The reason that this causes a problem is that at the Start Station of the Template on the Right Side the Template is good because the stationing of the alignment only has one possible solution, however on the inside the start station of the template on the alignment, when you offset a distance to the left, the station can be different because the projected point falls short of the break point in the alignment creating an ambiguous solution because of the angle join in the alignment causing the surface to not be created. You could see the surface in the Template Editor, but not in the corridor surface model and not in the Cross Section Editor.
By moving the start station for the Template up station slightly, the problem goes away. The video below shows the issue and the solution.