tl;dr (also didn't really see an explanation I'm the linked comments):

Tesseract dev hid a blocklist (compressed + base64 encoded iirc) of users and communities that should never be shown, sneakily inserted in an otherwise irrelevant commit.

There were a lot of communities/people on that list that didn't make a whole lot of sense, but the main issue is that it was a static list you (as the person hosting the front-end) have to opt out of.

db0 figured this out when he updated tesseract and suddenly no local posts appeared (the db0 instance is on the list).