Some explanation of what I'm thinking.
Co-Authors Plus uses a custom taxonomy called 'author' to retain relationships between posts and multiple users. The terms for the custom taxonomy have always had the user_login as the 'name' and the sanitize_title() version of the user_login as the 'slug'.
When we display the co-authors associated with your post, the lookup used to be using the term slug and then doing a get_user_by( 'login', $slug );. That now has become a lookup against the user_nicename value for the user.
If the user_nicename and user_login values differ at all, that might be a cause of the problem.