New users clicking category link see “access to this forum is by invite only” and cannot log in via OAuth2

I disabled Invite Only and created a group invite link for a private category. When new users click the link, they log in through our OAuth2 SSO, but they are not automatically added to the group—so they still cannot see the category.

Need help understanding why group invite links do not add new SSO users to the group, and how to ensure new OAuth2 SSO users are automatically added when using the invite link.