This happens to me as well. In most cases it seems to occur with routes where I might have forgotten to set next season's schedule properly.
I have also noticed setting a schedule for the full year, then clicking the 'clear schedule' button, does not clear the schedule for next season though. Only the current season. This is unintuitive and may be the cause for bug reports like this. I would suggest doing something about that design, and then seeing if this issue is reported again after that change.
I have also noticed setting a schedule for the full year, then clicking the 'clear schedule' button, does not clear the schedule for next season though. Only the current season. This is unintuitive and may be the cause for bug reports like this. I would suggest doing something about that design, and then seeing if this issue is reported again after that change.