Pleroma originally used heuristics to cover the only use-case of non-public Announces: peer to peer subscription (relaying).
Since then, the ability to have a hub-and-spoke relay architecture has been added, and a reference implementation made widely available.
But due to the heuristics this has sometimes resulted in relay traffic becoming visible to public, even though the relay does not address as:Public. Mastodon has fixed this in PR 9841, so we have no excuse not to fix it (they shouldn't be better at implementing our AP dialect than we are after all).