I added a class to teach my colonist to progress from low-tier proficiency into fire-arm proficient.
After the class is completed, the schedule is still stuck there. Now my colonist is idling at that time.

From hour 6 to hour 12, the schedule is stuck to that class. I cannot change it.

Workaround: Copy non-affected colonist schedule, and paste it into the affected colonist, will need to readjust the schedule to fit the old unbroken schedule.