Your issue can be quite frustrating as I have experienced it myself in the past and I know how it can be.

May I ask you. Do you by any chance use a Roles plugin or a plugin which hides certain areas of your website from certain groups or roles? 

If the answer is yes, then check on those plugins and it's configuration for each role, particularly the one role having the issue.

In my case the plugin was misconfigured to allow an inferior role to see and access an area they normally wouldn't be able to see. So they user was immediately being switched over to the Admin role in wPforo after logging in.

So make sure all levels of access are set correctly for each role in any of those plugins if you use them.

Hope this can give you some ideas on things to look for.

I hope this finds you well. We've observed an issue with the Wpforo plugin's user group assignment.

In our settings, the default usergroup for new members is set to “Customer.” However, we've noticed that new users are being assigned to the admin user group instead. While I did implement a workaround in the past to assign the 'Customer' role when users register through either Nextend or the native registration form, this hasn't fully addressed the problem. I've discovered that the affected users primarily seem to be those who haven’t completed the EDD checkout process.

I've attached a screenshot for your reference: 

To ensure consistent user experience, I kindly request your insight into why the Wpforo plugin might not be assigning the default user group to new members as expected.