Answer
Restart the user's client session entirely, rather than relying on an 'Update' or 'Reload Configuration' action — some metadata changes, like adding an attribute to a List of Values connected to a Custom Logical Unit, only take effect after a full session restart; refreshing or reloading configuration in place doesn't propagate this specific kind of change. Logging out and back in (or fully restarting the client) is what actually makes the new attribute populate correctly in the search window.