There is no such thing as “global search” in the fediverse. (Unless you use off-instance tools like lemmyverse)
Search can only return results that at least one local user has interacted with (meaning, subbed to).
The exception is when entering the url or exact name of a user or community. In these cases the server will actually contact the other instance, to establish a new federated connection. For a given community to appear on a given instance, at least one user must do this, and subscribe.
Until that happens, an instance will not receive incoming content related to that community, and hence it will not show up in search results.
Triggering this behaviour requires you to be logged in, and for the client to support detecting that you’re doing so, and sending the relevant, different, authenticated API call.
Additionally, if you are that first person, even once the community starts getting federated, it takes a while for the content to start coming over. Are you sure you were seeing “copies” and not just half-synced newly federated communities? Old comments and votes do not get synced when the connection is first started by a first subscriber from a given instance. Also, the subcount you see only tells you how many subs the community has from your instance, not globally.