In the MDL-70176, we fixed the issue for 'separate group filter' to show other group students.
This is an issue for 'separate group filter' to show a discussion topic for the other group.

Steps to replicate:

1. Login as admin.
   1. Create a course.
   2. Enrol 1 'Non-editing teacher' and 1 'Student' to the course.
   3. Create Group 1 and add the teacher and student.
   4. Create Group 2 and add the student.
   5. Create Group 3 and add the teacher.
   6. Create a forum with following options.
      • Common module settings > Group mode > Separate groups
      • Whole forum grading > Grade > Type > Point
   7. In the forum, choose 'Separate groups' to 'Group 1'.
   8. Add a new discussion topic 'Discussion 1' to 'Group 1'.
   9. Choose 'Separate group' to 'Group 2'.
   10. Add a new discussion 'Discussion 2' for the group.
   11. Choose 'Separate group' to 'Group 3'.
   12. Add a new discussion 'Discussion 3' for the group.
2. Login as the student.
   1. Go to the forum.
   2. Choose 'Separate group' to 'Group 1'.
   3. Click the 'Discussion 1', and reply to the discussion.
   4. Choose 'Separate group' to 'Group 2'.
   5. Click the 'Discussion 2', and reply to the discussion.
3. Login as the teacher.
   1. Go to the forum.
   2. Choose 'Separate groups' to 'Group 1', and click 'Grade users'.
   3. Choose the student.
   4. Teacher can see 'Discussion 1' and the student's post.
   5. Teacher can also see the 'Discussion 2' but only title. 'Discussion 2' is for 'Group 2'. The 'Separate groups' filter doesn't work properly.