It seems you found the issue.
Convoys without any stop in their route do not make much sense, so I'm not sure if this is a bug or not, but I got it reproduced.
In any case, for the next bugreport, please make sure to always attach a savegame in which it can be reproduced and a detailled description how to reproduce it.
In some cases either of the above might be sufficient.
Although, a minimal reproduction case with a description is always prefered.