Probably something went wrong with the initial federating of the community.
Communities don't appear off-instance on a given instance until at least one user on that other instance has deliberately searched up that specific community, and subscribed to it.
When that occurs, the community gets synced to the other instance, and future updates are federated to other instances with subscribers as they come in, provided there is at least one subscriber on the other instance. This way every server does not need to host a copy of every community on every server. Only what the users on it actually want to see.
If the subcount drops back to zero (from that other instance), the federating stops.
Without a subscriber, the content is not available on the other instance.
The community seems to be working, but the past content has not synced the way it is supposed to. At this point, if it hasn't, it likely never will. Future updates should work though.
You can also trigger federation manually by putting the url of a post or comment you want to interact with in search, while logged into your own instance.