[Domain Mapping] Cannot map subdirectory subsite to subdomain

We have a subdirectory multisite installation set up at mydomain.com/blog. We've set up a subsite mydomain.com/blog/subsite and we're trying to map this to subsite.mydomain.com. We've created a subdomain in cPanel and pointed its document root to the multisite installation root.
But we cannot get the mapping to work and the domain mapping health status keeps saying invalid. Please advice.