Hi return0

This means that at some point Googlebot discovered https://siteB. It could simply have been a misconfigured CMS, or a bad sitemap, or an errant link one of your visitors shared on a forum, or something a previous owner of the domain did, or anything really. You may think there are no links to the site, and that may be true right now, but it's about something that Googlebot found in the past.

The correct fix is, as you say, to make sure the server doesn't respond to invalid certificate+site combinations.