Ajax : textarea vs text_format

Hello everyone,

In a form, I want to save the content of a field when leaving this field, without any submit button.
The following works perfectly while the field type is textarea but it doesn’t work anymore when I change to text_format type.
Why is that and how can I fix it ?

Thanks for your help,

public function buildForm(array $form, FormStateInterface $form_state) {
$form['journal'] = array(                                     
  '#type' => 'textarea',
  //'#type'=>'text_format',
  '#title' => 'Journal',
  '#ajax' => [
    'callback' => '::saveJournalAjax',                             
    'event' => 'change',
    'progress' => array(                                           
      'type' => 'throbber',                                        
      'message' => t('Saving...'),                         
    ),
  ],
);
return $form;
}
public function saveJournalAjax(array $form, FormStateInterface $form_state) {
$ajax_response = new AjaxResponse();
// saving process
return $ajax_response;
}

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