When I try to search in a collection, the search brings up no results. The same search done in either My Library or a Subcollection does bring up results in the usual way.
That would indicate that the item is not a part of the collection (but only of the subcollection). If you want to show all items in subcollections in the collection as well, enable the "Recursive Collections" hidden preference: https://www.zotero.org/support/collections_and_tags#recursive_collections
https://www.zotero.org/support/collections_and_tags#recursive_collections