If a citation is made and saved in Paperpile, and the user later goes back and changes the BibTeX key, it does not show up when searching by that new key. If the user makes a copy of the citation, then the citation is searchable by the key. So I guess updates to the BibTeX key are not getting indexed properly for search purposes on an existing record.
I like to create manual citation keys, but once I do the record is no longer searchable by the key so I have to copy the record again. I presume this is a bug and the software’s intention is for updates to fields to maintain searchability.