We changed the Admin Email address on the Config > Monitor page and stopped and restarted the monitor. Most admin emails now go to the new address, but the log purge emails that are generated as per automatic log purge settings configured on the Config > Logs page continue to be sent to the old address.
The log purge is handled by a separate process that does not read the updated email configuration until it's restarted.
The problem can be resolved by saving an Update to the Automatic Log Purge Settings configuration on the Logs > Config page. It is not necessary to actually make a change to the configuration, just hitting the Update button should work. Look for a message "logwatch[<PID>]: Starting up logwatch" in the session logs to confirm that the process was restarted.