Embedded AJAX responses aren't application/json, but application/vnd.drupal-ajax

To send the final values for BigPipe placeholders, we send Ajax commands. Those are sent in <script type="application/json"> tags. But it'd be better/more accurate to use <script type="application/vnd.drupal-ajax">.

That's also the Content-Type header Drupal 8 sends when sending Ajax responses. Since BigPipe is sending embedded Ajax responses,this is IMO a logical consequence/improvement.