Try this
function my_relationship_result($title, $post, $field, $post_id) {
$args = array(
'fields' => 'names'
);
$terms = wp_get_post_terms($post->ID, 'area', $args);
if ($terms) {
$title .= ' ['.implode(', ', $terms).']';
}
return $title;
}
add_filter('acf/fields/relationship/result', 'my_relationship_result', 10, 4);