Forcing https on primary domain breaks new Pro Site subdomain URLs

I currently have 1 SSL cert applied to my primary domain and I'm forcing https on this primary domain, but none of the subdomains. I'm not looking to purchase a wildcard SSL cert at this time so I'm keeping the subdomains as http.

After a user registers for a site and the credit card transaction clears, the Admin URL that is presented to them is https which causes a 404 error since none of the subdomains have a SSL cert. I also noticed when I do a Network Upgrade, all subdomains fail to upgrade since it's pointing to https.

How do I fix this behavior and point to http for the subdomains?