multisite https redirects to http home_url

Did you change the urls to https in Network Admin–Sites–edit an indiv site? Not just at the info tab, but also in Settings?

That might help you find any places where http is still lurking.

I think you should be able to do individual sites like you are proposing, but maybe the whole network needs to be https?

Have you verified that the wildcard SSL is installed correctly? Maybe create a subdirectory folder in your hosting, and just put a plain html page there and see if it displays HTTPS?

If that works, what about forcing the whole network to be HTTPS in the wp-config? Even if that’s not what you want to do eventually, it might be a good test to see if it helps get some additional clues about where the problem is.

OR, maybe it’s not an SSL related problem at all, and really just related to changing the domain/urls? This might offer a clue: https://ww.wp.xz.cn/support/topic/wordpress-multisite-sub-sites-all-redirect-to-the-main-site?replies=9

I’ve only ever installed SSLs on single sites, so take all this with a grain of salt. 🙂

You said you changed the urls to HTTPS in the database and on the settings tab in Network Admin. Have you checked in the subsite dashboard General, to make sure that the WordPress Address (URL) and the Site Address (URL) are both HTTPS?

Also, have you tried explicitly redirecting the subsite in question via htaccess rules?

You can also check via your network dashboard. If the sub-site in number 5 then looks at that site’s edit page.

http://mainsite.com/wp-admin/network/site-info.php?id=5

And confirm that the Site Address (URL) has https://subsite.mainsite.com/ which should be the case.

If that doesn’t work then looking at the curl output may help.

For example on my subsite with a valid TLS cert:

$ curl -I https://subsite.mainsite.com/
HTTP/1.1 200 OK
Server: nginx/1.10.0
Date: Sat, 28 May 2016 20:02:18 GMT
Content-Type: text/html;charset=UTF-8
Content-Length: 41076
Connection: keep-alive
Last-Modified: Sat, 28 May 2016 13:21:50 GMT

Which is what I expected. But if the URLs didn’t match up then I’d expect something like this.

$ curl -k -I https://subsite.mainsite.com/
HTTP/1.1 302 Found
Server: nginx/1.10.0
Date: Sat, 28 May 2016 20:03:45 GMT
Content-Type: text/html; charset=UTF-8
Location: https://mainsite.com/wp-signup.php?new=subsite

I’m using nginx but the same results should happen with Apache2.