Can't get www to prefix on a mapped doing when I'm using SSL

I have a mapped domain, a few actually, set up on a multisite installation.
I've set the IP to point to the proper place and everything resolves as it should *except* if I try to map, it rolls back to

I can set www. with http just fine, just not with https.

I'm on SiteGround using Let's Encrypt with add-on domains.