Why do WordPress emails to multiple recipients include \n in the list

As you mentioned, it could be a plugin. Tracking that down would require deactivating other plugins and retesting to find out. Tracking down the actual problem is really what you need to do.

However, in absence of actually finding the problem, you may be able to filter out the new line indicator with the following filter (add it to your theme’s functions.php):

add_filter( 'wp_mail', 'my_wp_mail_filter' );
function my_wp_mail_filter( $args ) {

    $args['to'] = str_replace( '\n', '', $args['to'] );

    return $args;
}

This should take the “to” address and run str_replace() to replace any instances of “\n” with an empty value.