CourseManager::unsubscribe_user() deletes resources equally from or out of session
The unsubscribe_user() method, if provided a session id, should delete only stuff related to this session.
This is set as urgent although it is extremely rare to have students subscribed to a course both outside and inside of a session, but because it might so happen that a user is subscribed to 3 sessions, then is unsubscribed from one of these session only: this would also delete his results in other sessions.
In this case, it seems like the code would delete all the contents of the student in the previous session, which is probably not desired...
We need the basis of v10 to be ready before we can fix this.