Despite these communities existing, Boost doesn't load them directly. I can still see posts from them in Local view, but viewing the community directly doesn't work. This happens on multiple different instances.
The (SFW) community in the image is: !announcements@burggit.moe
burggit.moe/c/announcements
(I'm not sure which link style works, so I'm including both)
The api sometimes returns 404s if you try to get a community that hasn't been explicitly searched / federated by the instance yet. Not sure how the app deals with it but things are certainly more polished than my own work :โข|
You might have to search for the community in the search bar until it federates and then access it (either in the app or on the webpage)
Alternatively the app could use the federate object API function