You can try adding this markup, but my bet is that the issue will resolve itself after some time, especially since Discourse has normally all the required tags used by Google to create a site title.
More like Google being silly.
Searching for “wrong site name google” on Google returns many, many similar issues.
As for me, it was a sports club forum, and the site title was wrongly set to “Dr-Ed-Mariano”, which had nothing to do with the forum, wasn’t even mentioned in the whole site.
Very, very weird, and the forum was many years old with no domain change.
My phpBB forum lacked the main tags used by Google to create the site’s title and Google algorithms probably messed up the title creation.