Steps to reproduce:
- Login as admin.
- Create a course with 'No groups' in 'Group mode'.
- Enrol admin and 2 more users to the course.
- Create a group with an image and add the admin user there.
- Create a forum with 'Separate groups' in 'Group mode'.
- Post a message to the forum, only for the group.
- Access to the main page of the forum (where the new discussion will be displayed).
- 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.