I suppose that „aggregation“ you’ve seen in the search results is based on the auto-created collections having the same online metadata guid. So Plex might be treating it the same as e.g. different-quality versions of the same movie in different libraries or different editions of a movie in the same/different libraries.
That guid is not editable in the user frontend, so you cannot currently „force“ Plex to treat manually created collections to be considered as „the same“ (despite showing same-named collections in different libraries as „related items“.
There’s already an existing suggestion asking for an option to „match“ manually created collections, therefore deriving their respective descriptions, posters… and guids from The Movie Database. I consider this would also achieve what you’re looking for.
