add_filter('acf/save_post', acf_save_post, 10, 1);

function acf_save_post($post_id) {
  $new_post = get_post($post_id);
  if (!($new_post instanceof WP_Post)) {
    return;
  }
  if (!have_rows('modules', $post_id)) {
    return;
  }
  ob_start();
  global $post;
  $post = $new_post;
  setup_postdata($post);
  get_template_part('includes/modules', null, array('save' => true));
  $html = ob_get_contents();
  ob_end_clean();
  wp_reset_postdata();
  $post = get_post($post_id);
  $post->post_content = $html;
  wp_update_post($post, [], false);
}