Forum Grading Does Not Respect Separate Groups Filter

Steps to replicate:

1. Login as admin.
2. Create a course with 'No groups' in 'Group mode'.
3. Enrol admin and 1 more users to the course.
4. Create Group 1 and add the admin user there.
5. Create Group 2 and add another user there.
6. Create a forum with 'Separate groups' in 'Group mode' and 'Whole forum grading > Grade' set to 'Point'.
7. Navigate to the forum.
8. Leave a post as an admin in Group 1.
9. Login as a user and leave a post in Group 2.
10. Login as an admin and navigate to the forum.
11. Choose 'Group 1' in 'Separate groups' dropdown.
12. Click 'Grade users'.
13. See that both posts are displayed.
14. Close grading page and change 'Separate groups' to 'Group 2.
15. Click 'Grade users'.
16. See that both posts are displayed.