This problem arose from WP Support request Addressing the Author Terminology and Owner Filter Issue.

The Author metabox allowed the use of a filter on the parameter WHO which allows only Authors to be listed in the metabox.

The parameter who has been withdrawn for a while so the filter can serve no purpose.

Currently all users are listed.

This list should only contain the list of users with 'edit_documents' permissions since assigning documents to people without the capability means that that user cannot manage the document.