Record(s) not linked to an agent record appear in agent's Linked Record display in staff interface

This issue is related to https://archivesspace.atlassian.net/browse/ANW-742 and https://archivesspace.atlassian.net/browse/ANW-783. The attached screenshots show two distinct agents which have the same name. One of the agents is linked to a resource, the other is not. However, the resource that the one agent is linked to appears in the Linked Records display for both agents. Only the actual linked agent appears in the JSON for the resource record, and only the linked agent record includes a value in the "used_within_repositories" field, etc., in its JSON record.

As notes in comments to https://archivesspace.atlassian.net/browse/ANW-783, a search is being used to populate the Linked Records display, which leads to incorrect display of linked records.