Non-wordpress subdomain in https redirects to main wordpress domain

I wasn’t sure where to post this as my WordPress install is a Multisite, but this only concerns my problem indirectly (or at least I think so…)

I also tried posting this on stackoverflow but I’m not getting any answers and this is starting to be urgent (this is for a client).

I’ve got my WordPress installed as a multisite network on the main domain – let’s say https://www.domain.com.

I also have subdomains set at store-ca.domain.com and store-us.domain.com (no www), on which I have an Opencart multistore.

I want to be able to use the checkout section of my stores in SSL, so I got a wildcard certificate installed so it would cover both subdomains.

If I simply add an ‘s’ to http from one of the subdomains, no matter where I am in one of the stores, I’m redirected to https://www.domain.com, in simple http. And the fact that it’s an Opencart install doesn’t seem to matter, as I also made a test folder with a simple index.html file saying “hello”, created a subdomain to direct to it and tried accessing it through https; same result, I’m redirected to the WordPress site.

It doesn’t seem to have anything to do with either system’s htaccess files as I wipe them out and the redirect still happens (and the pretty URLs stop working, as expected).

So, anyone have any idea what could be causing this?