Can't toggle forced schema in Domain Mapping plugin

I have the Domain Mapping plugin installed on our WordPress (4.7.1) multisite. The plugin works as expected, however I can't force HTTPS on some of our domains.

I can force HTTPS on the original domain for login/admin and front end pages. It works as expected. However, when I go to the Domain Mapping settings on most of our other sites and either click the key next to the mapped domain (Toggle forced schema) or delete then add the HTTPS version of the domain, the changed doesn't get "saved".

I say "saved" because in the admin area, after refreshing the page, the domain reverts back to HTTP, however the change IS being saved in the Domain Mapping table in the database.

I have recently purchased a multi-domain certificate and I've installed this correctly. I manage the server via Plesk. WordPress is setup on the original domain and all the other domains are setup as aliases of the original domain (hence the multi-domain certificate).

Do you have any suggestions or need any more info?

Many thanks,