That's how federation is supposed to work. If one user searches content, you get it once. If a user of your instance subscribes to a community, you get updates for that community.
The point is to reduce load on your instance by not federating content that nobody on your instance needs.
It's based on a flawed understanding of how communities work, specifically that reddit-style forums only work well when you have huge communities that everyone sibscribes to. So, most instances end up with most content anyway.
Also, keep in mind, all conent that has been federated to your instance is on your srver and thus legally your responsibility. If there's illegal content on there, you are liable if you don't delete it.