Steps to reproduce the effect:
1. The teacher creates a course with a single activity (ex. CHOICE)
2. The teacher sets the completion conditions to "View the activity" and "Make a choice"
3. The student sees the two criteria in the "To Do" menu on the right side of the activity.
4. The student opens the activity (view) but does not make any choice.
5. The student sees the activity marked as viewed
6. The teacher manually marks the activity in the "Activity completion" report.
7. The student sees both conditions marked in green in the "To Do" menu.
8. The teacher deletes the previously completed manual completion.
9. The student sees both conditions as incomplete (which is obviously half correct).

The error is caused by the value of the "override" field from the {course modules completion} table. The field will remain with the value of the teacher's userid until the student makes a choice.
Then instantly both conditions (vies & choice) will be marked with green.