The main site of multisite is set to use SSL, so SiteURL and Home URLs are properly set to https://. However, when you create a new sub-site (via back-end, as super-admin but in any other case too), that sub-site is by default created with "http://" prefix.
- main site is https://domain.com
- go to Sites -> All -> Sites and [Edit] the main site
- you'll see that both SiteURL and Home URL start with https://
- now create a subsite and [Edit] (on "Sites -> All sites")
- you'll notice that both SiteURL and Home URL start with http://
How to force that these new sites are automatically created with https:// protocol prefix right away?