I just found a related problem: I saw two carriages approaching a town, both empty, with the town being the last stop in their mirrored schedule, and manually sent one of them to the first stop instead. Which is a town more than 30km away, and also where the nearest depot is located. That resulted in an error message about range limits, and another about "couldn't find a depot". The convoy teleported to the depot anyway, but lost the schedule.