Thank you for your answer.
I do not understand, maybe my explanations are not good 🙁
My solution is to update the post (image) when the rwmb_frontend_after_process is activated.
$arg = array('ID' => $media_id, 'post_parent' => $post_id,);
wp_update_post( $arg );
I wonder if this behaviour shoulb be automatic or if i have a bug in my code somewhere.
Have a nice day !