Steps to reproduce:

1. Login as admin.
2. Create a course with 'No groups' in 'Group mode'.
3. Enrol admin and 2 more users to the course.
4. Create a group with an image and add the admin user there.
5. Create a forum with 'Separate groups' in 'Group mode'.
6. Post a message to the forum, only for the group.
7. Access to the main page of the forum (where the new discussion will be displayed).
8. Click over the group image.

Expected behaviour: the participants page should be displayed with the group filter enabled (so only admin user should be displayed).

Current behaviour: the participants page is displayed, but no group filter is applied.

If the course group mode is changed to 'Separate groups' or 'Visible groups', it works as expected.