how to reinitiate a default field value every time the content edition is loaded?
You need to use hook_form_alter 

    use Drupal\Core\Form\FormStateInterface;

    /*
     * Implementation of hook_form_alter()
     */
    function mymod_form_alter(&amp;$form, FormStateInterface $form_state, $form_id) {
      if($form_id == 'node_evenement_edit_form'){ // target a specific form
        $form['field_notif']['widget']['#default_value'] = 0;
      }
    }