diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index a0ba52c..8581198 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -604,9 +604,11 @@ public function processForm($form_id, &$form, FormStateInterface &$form_state) {
   }
 
   /**
-   * #lazy_builder callback; gets an actual path for the path.
+   * #lazy_builder callback; this renders a placeholder that's used in a form
+   * action to use the current request's url
    *
-   * @return string
+   * @return array
+   *   A renderable array representing the form action.
    */
   public function renderPlaceholderFormAction() {
     return ['#link' => $this->buildFormAction()];
@@ -626,7 +628,11 @@ public function prepareForm($form_id, &$form, FormStateInterface &$form_state) {
       $placeholder_render_array = [
         '#lazy_builder' => ['form_builder:renderPlaceholderFormAction', []],
       ];
+
+      // Attach the lazy builder to the form and add the placeholder in the form
+      // action so the #lazy_builder callback knows what to replace.
       $form['#attached']['placeholders'][$placeholder] = $placeholder_render_array;
+      $form['#action'] = $placeholder;
     }
 
     // Fix the form method, if it is 'get' in $form_state, but not in $form.