Can't submit data when rendering a webform programmatically
@see How to embed a Webform? 

I recommend using something like...

return [
  '#theme' =&gt; mycontroller_page',
  '#content' =&gt; [
    'title' =&gt; t('I am a page from your controller'),
    'form' =&gt; [
      '#type' =&gt; 'webform',
      '#webform' =&gt; 'my_webform_id',
    ],
  ],
];