Hi @ecommjuice . You might also want to check your settings at:

1. Forum Dashboard
2. Forum Settings
3. Then scroll down at the very bottom, you will see the settings for "Display Usergroup under Post Author Avatar" and make sure the "Registered" Box is also checked.
4. Don't forget to click the "Update Options" blue button.

5. Then refresh your page or press Control F5.

Maybe that was why when you changed the role of those missing "members" from the list to "Admin", then they appeared.

6. After you have done the above steps, you can change the roles back from admin to "Registered" for these members, 
via WP Dashboard
Users
Select Username
Then change "Role" back to "Subscriber" ( WP "Subscriber" = to "Registered" in wpForo)

Under that you will see the explanation: 

Forum - Usergroup
Forum Usergroups are synched with User Roles based on the Role-Usergroup synchronization table. When you change this user Role the Usergroup is automatically changed according to that table.

7. Click "Update user" .

Hope this helps.  Good luck and God bless you.  :-) 

P.S. Also, I noticed from your screenshot that your WordPress needs an update. There might be some plugins that need to be updated as well. :-)

Please navigate to Dashboard > Forums > Usergroups admin page, edit the Usergroup these members belong, make sure the "Display on Members List" option is enabled. Then delete all caches and check again. 

There are 3 members that do not appear in the public member list. But if you search in the search engine, they appear.

I have reviewed the configuration of those 3 users and it is exactly the same as the rest. I have cleaned the cache and it keeps happening.

Could you help us?