Hello!
You most likley ran out of design profile. If the alignment continues, and the profile ends "in the middle", the corridor will drop. Either modify corridor end station to correspod with profile, or modify profile to the end of alignment/corridor.
Iggy
AutoCAD Civil 3D Certified Professional
This happens so much if only C3D could warn of exceeding design profile limits when clicking OK on corridor building.
As far as the middle of the corridor I agree. I have had my design profile where two-line segments do not connect/broke, and it will build this way. Easiest way is to notice that profile labels have stopped somewhere along the way.