That's odd, usually I would say it's because it's a group site and you need to be made a member for search to show up correctly but that doesn't seem to be the case here. Is this communication site stand alone or part of a hub?
Also there used to be an issue where removing users permissions from the root site collection at tenant.sharepoint.com would cause index issues, you didn't remove permission off that site have you? Figured it wouldn't matter because you are admin, but if you don't have explicit permission it could maybe be it.
Don't know if it relate, but I have a support ticket opened because I'm not getting results when I search for words that are in Site titles/urls. Search has been been very unreliable for me for the past few months. I'm starting to wonder if some of the efforts to modernize search using the Graph are somehow impacting the classic FAST search features.