This is a bug report. The domain mapping plugin doesn't seem to be redirecting properly if in the domain mapping settings are set to force the mapped domain in http on the frontend when the original domain is https (see screenshot).
In the domain mapping setting it's set for "http" and Front end redirect should be "directed to mapped (primary) domain"
See attached screenshot for details.
When entering the original domain in http this will properly redirect to the mapped domain. But when entering the original domain in https AND https is forced network wide on the original domain (*.example.com) it won't redirect to the mapped domain, it'll just go to the https original domain name.
So with the above settings and https forced network wide on the original domain (i.e. on all subdomains):
If I type in http://subdomain.example.com I properly get redirected to http://mappeddomain.com
Network admin settings:
Login mapping: original domain
Cross-domain autologin: Yes
Would you like to force https in login and admin pages: Yes
Would you like to force http/https in front-end pages: No
I hope I explained that well enough, let me know if there are any questions.