function my_cpt_title_update( $post_id, $post, $update ) {
$title = array();
if( get_post_type() == 'cpt' ) {
$title = get_field('acf_variable', $post_id);
}
$content = array(
'ID' => $post_id,
'post_title' => $title
);
remove_action('acf/save_post','my_cpt_title_update', 20);
wp_update_post( $content );
add_filter('acf/save_post', 'my_cpt_title_update', 20);
}
add_action('acf/save_post', 'my_cpt_title_update', 20);