How to reproduce the problem:

1. Create a course with groups.
2. Enrol a student that doesn't belong to any group.
3. Create a workshop configured to use Visible groups.
4. Move the workshop to submission phase.
5. As the student, enter the workshop and add a submission.
6. Call the WebService mod_workshop_get_submissions with the student's token, passing the workshopid and groupid=0 (or don't include the groupid, it's the same). Check that you don't receive the student submission.

Adding the student to any group fixes the issue.