"Assignee" and "reporter" pickers in issue search expose user data

We're an agency with multiple clients on JIRA. Similar to JRA-7889, when any of our users search for issues and use the "assignee" or "reporter" dropdowns, they can see all other users of the systems which is effectively a full list of our clients.

This is a major security problem for us, as it exposes our whole client list, with their emails, to every user.

We need a way to select that people only see certain other people (their coworkers, and us).