Secure content now confused between main domain and subdomain

I have a main WP site (call it mysite.com), and a blog WP subsite (blog.mysite.com).

Both were working pretty good, but the blog site had some insecure links, so I installed SSL Insecure Content Fixer, and activated it with the Simple configuration. It cleaned up one of 17 issues, and then I figured out where the rest of the https:// calls were and cleaned them up manually. https://blog.mysite.com now ran securely and perfectly. Cheers.

But then I found that when I called https://mysite.com, which had worked great before, now it was bringing up page content from blog.mysite.com. The browser URL bar would still show https://mysite.com — it didn’t seem to be redirecting according the URL bar — but it was displaying blog homepage content.

I deactivated SSL Insecure Content Fixer and tried doing a bunch of stuff to fix it: restored the previous day’s WP database for mysite.com, restored program files from the previous day, and had my host check both SSLs for both the main site and the subsite. They found no problems. I flushed the cache and deleted cookies for both sites in my browser. I tried multiple browsers. Nothing has helped.

If I run unsecure https://mysite.com it pulls up the right WP content. But if I run secure https://mysite.com, it pulls up wrong WP content (it pulls up the blog’s main page).

If I try to securely browse a page on the main site (e.g. https://mysite.com/anypage/) I get the blog’s 404 “This is embarrassing” page content.

Ideas?