mod_forum: Unable to post in Forum to All participants when using Visible Groups

An error message is displayed when user who is a member of a group attempts to Post to a Forum "All participants" when group mode is set to Visible groups.

Environment:
Moodle - 3.9.3+ (Build: 20210108)
Forum - 2020061501

Replication:

1. Create a course.
2. Enrol four (4) student users.
3. Create two (2) groups and add members
   • groupA - user1
   • groupB - user2, user3, user4
4. Turn editing on.
5. Add a Forum activity and set Group mode to Visible groups.
6. Log in as user1, go to the course and visit the Forum activity.
7. In the "Visible groups" drop-down control, select "All participants".
   RESULT: The following message is displayed: You are not able to create a discussion because you are not a member of any group.