enabling CDSSO creates double login when domain is mapped.

Issue: enabling CDSSO creates double login when domain is mapped and using custom login page.

I have a custom login page via a shortcode. When using domain mapping and having CDSSO enabled with async on, the user is asked to login on their domain (via the page with the shortcode) and then redirected to the orginal domain and asked to log in again.

This is unexpected. Logging in once should log them into both the original and mapped domain. How can I fix this?