Catalog items are not being returned when searched for on the Portal.
This issue was seen because, on the affected instance, the sc_cat_item - extending from sys_metadata was not indexed completely.
Updating a record from the sc_cat_item table triggered indexing for that item and it was searchable.
The way to resolve this issue is to reindex the affected table - sc_cat_item or sys_metadata as required.
This can be done without impacting existing search results by using real-time indexing which is described in the document at the link below: