acf/save_post

// UPDATE: this code is working. Just FYI if you come across this link through https://support.advancedcustomfields.com/forums/topic/save-edited-post-as-pending-add-acf-field-title/

function change_post_status() {

    if ( isset($_GET['ad']) ) {
        $post_id = $_GET['ad'];
    } else {
        $post_id = $post_id;
    }

    $site_url = site_url();

    global $wpdb;
    if ( ! $post = get_post( $post_id ) ) {
        return;
    }
    if ( 'pending' == $post->post_status ) {
        return;
    } else { // is post != pending
        $entered_title  = $_POST['acf']['field_57dec9df66a5b'];
        $cleaned_title  = preg_replace('/[^A-Za-z0-9\-\s]/', '', $entered_title); // Removes special chars.

        // Update DB with pending status.
        $wpdb->update( $wpdb->posts, array( 'post_status' => 'pending', 'post_title' => $cleaned_title ), array( 'ID' => $post_id ) );
        update_field('field_57dec9df66a5b', $cleaned_title, $post_id);

        // send email
        $to         = get_option('admin_email');
        $subject    = 'Published ad is edited';
        $headers[]  = 'From: DVP Portal <support@dvp.nl>';
        $headers[]  = 'Content-Type: text/html; charset=UTF-8';
        $message    = '<a href="'. $site_url .'/wp-admin/post.php?post='. $post_id .'&action=edit">Klik hier voor de ad</a>';
        wp_mail( $to, $subject, $message, $headers );

        // clean the cache for the post
        clean_post_cache( $post->ID );

        // Trigger all transition hooks in wordpress (in case other functions hook into those to do things.
        $old_status         = $post->post_status;
        $post->post_status  = 'pending';
        wp_transition_post_status( 'publish', $old_status, $post );

    }

}
add_action('acf/save_post', 'change_post_status', 20);