Under certain circumstances workshop_get_user_assessments may fail

When a student access a workshop in view, under certain circumstances (that are still under investigation and not always reproducible) get the following error:

workshop_list_student_submissions: unable to get submission

Trying to identify the problem we displayed submission id and user id as follows:

workshop_list_student_submissions: unable to get submission id 694 for user 5709

According to db tables, submission id 694 should not be retrived for this student, thus the error.

Problem appears only for some students, while others does not suffer the issue on the same workshop.