diff -u b/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php b/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php
--- b/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php
+++ b/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php
@@ -45,8 +45,12 @@
    *   The event to process.
    */
   public function onView(GetResponseForControllerResultEvent $event) {
-    // Ensure this request is rendered as HTML so that we can manage the
-    // creation of an AJAX response directly in self::onException().
+    // To support an AJAX form submission of a form within a block, make the
+    // later VIEW subscribers process the controller result as though for
+    // HTML display (i.e., add blocks). During that block building, when the
+    // submitted form gets processed, an exception gets thrown by
+    // Drupal\Core\Form\FormBuilderInterface::buildForm(), allowing
+    // self::onException() to return an AJAX response instead of an HTML one.
     $request = $event->getRequest();
     if ($request->query->has(FormBuilderInterface::AJAX_FORM_REQUEST)) {
       $request->query->set(MainContentViewSubscriber::WRAPPER_FORMAT, 'html');