This will need to be substantiated by someone who knows this for sure, but I believe the way this works is that when you search for a community that's not on your home instance from in the app, the app asks your home instance if it knows any communities that match the search string. If your home instance has never loaded the community you're looking for before that time, it won't come up in search, because your home instance doesn't have it federated yet.

However, when you click on the community link, the app is asking your home instance to load from , and so your home instance will first federate with (read: subscribe to) that community, then return the results to your app.

Your home instance now knows about that community, and should now be able to return it in search.