Ajax submission not working for Webform within a node
This is a gotcha of AJAX forms.
When you load an AJAX form via AJAX it loses its AJAX functionality.
In order to keep it working, for the form that is getting loaded via ajax, you need to add a url and options keys to #ajax
/*
 * Implements hook_form_alter()
 */
function mymodule_form_alter(&amp;$form, FormStateInterface $form_state, $form_id) {

  // I created a webform called &quot;test&quot;
  if ($form_id == 'webform_submission_test_add_form') {
    $webform_id = 'test';
    $form['actions']['submit']['#ajax']['url'] = \Drupal\Core\Url::fromRoute('entity.webform.canonical', ['webform' =&gt; $webform_id]);
    $form['actions']['submit']['#ajax']['options'] = [
      'query' =&gt; [
        Drupal\Core\Form\FormBuilderInterface::AJAX_FORM_REQUEST =&gt; TRUE,
      ],
    ];
  }

}


Side note: this loss of AJAX issue doesn't occur with the HTML way of opening dialog boxes
&lt;a class=&quot;use-ajax&quot; 
   data-dialog-options=&quot;{&amp;quot;width&amp;quot;:600}&quot; 
   data-dialog-type=&quot;modal&quot; 
   href=&quot;/form/test&quot;&gt;
   Should load Test webform in a Modal
&lt;/a&gt;

because it has the URL of the form and knows its an AJAX request from the class name.