This is a simple software bug. You see, the way Lemmy federates is that updates to a community only federate to instances that have a user subscribed to that community. If you take a community like [email protected], I’m the only person on all of Aussie Zone who subscribed to it. I know it because when I subscribed, we started getting updates from Blahaj.Zone about that community.
A few days ago, I was mistakenly banned from that community. And this caused Blahaj.Zone to stop sending our instance updates on the community. I spoke with the admin of the community and got unbanned, but the unban didn’t federate. The instance will only let us know I’m unbanned if someone here subscribes over there. And I can’t subscribe, because this server thinks I’m banned. It’s a deadlock. If it wasn’t for the ban, I could subscribe and the server would realise there isn’t a ban anymore. But I can’t, because this server thinks there is still a ban.
Ha! That’s a wacky edge case. Federation is instance based though. That explanation only makes sense if no aussie.zone user subscribes to any lemmy.blahaj.zone community. Given the overlap of users (a few blahaj users are regulars on aussie.zone), I have a hard time believing that’s the case.
If the unban still hasn’t federated, try getting their mod to ban/unban again. We’ve had issues with posts in /c/Melbourne getting unpinned for other instances previously. The fix was to pin again, wait a minute or so and unpin.