When a teacher creates a group and set the membership visibility as "Hidden" they expects to see the hidden group in the "Separate group" dropdown menu, in the forum.

The help text for the "membership visibility" in the group setting states  "the hidden status makes the group visible only for teachers and it's members".
Either the help text is wrong or it's not working as intended.

Steps to reproduce (as admin or teacher):

1. Create a course.
2. Create a couple of groups
3. Edit the first group to set it's "Group membership visibility" to "Hidden"
4. Create a forum Activity with the Common module setting "Group mode" to "Separate groups" or "Visible groups"
5. In the forum index try to select the first group in the "Separate groups" dropdown
6. The group is not in the list, but it should, the help text for group membership visibility state that the hidden status makes the group visible only for teachers and it's members