Add the following to functions.php
when you are meddling and remove it after, or comment it out:
function no_mod_wpse_136092($data,$postarr){
if (empty($postarr['ID'])) return $data;
$old = get_post($postarr['ID']);
if (!empty($old)) {
$data['post_modified'] = $old->post_modified;
$data['post_modified_gmt'] = $old->post_modified_gmt;
}
return $data;
}
add_action('wp_insert_post_data','no_mod_wpse_136092',10,2);