fixed

src/Element/ComponentForm.php

@@ -72,9 +72,46 @@ class ComponentForm extends ComponentFormBase {
         [$class, 'validateFormElement'],
       ],
       '#theme_wrappers' => ['form_element'],
+      '#after_build' => [
+              [$class, 'afterBuild'],
+      ],
     ];
   }
 
+  /**
+   * Alter the element after the form is built.
+   *
+   * @param array $element
+   *   The element.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The form state.
+   *
+   * @return array
+   *   The altered element.
+   *
+   * @SuppressWarnings(PHPMD.UnusedFormalParameter)
+   */
+  public static function afterBuild(array $element, FormStateInterface $form_state) : array {
+    if ($form_state->isProcessingInput()) {
+      // For browser-submitted forms, the submitted values do not contain
+      // values for certain elements (empty multiple select, unchecked
+      // checkbox). Child elements are processed after the parent element,
+      // The processed values, stored in '#value', are bubbled up to the
+      // parent element here and then copied to the form state.
+      if (isset($element['#value'])) {
+        if (isset($element['slots']) && isset($element['slots']['#value'])) {
+          $element['#value']['slots'] = $element['slots']['#value'];
+        }
+        if (isset($element['props']) && isset($element['props']['#value'])) {
+          $element['#value']['props'] = $element['props']['#value'];
+        }
+      }
+    }
+    // Recopy the value to the form state.
+    $form_state->setValueForElement($element, $element['#value']);
+    return $element;
+  }
+
   /**
    * {@inheritdoc}
    */
@@ -309,7 +346,6 @@ class ComponentForm extends ComponentFormBase {
       return;
     }
     if (isset($element["#component_validation"]) && !$element["#component_validation"]) {
-      $form_state->setValueForElement($element, $element['#value']);
       return;
     }
     try {
@@ -326,7 +362,6 @@ class ComponentForm extends ComponentFormBase {
         $renderer->executeInRenderContext($context, function () use (&$build, $renderer) {
           return $renderer->render($build);
         });
-        $form_state->setValueForElement($element, $element['#value']);
       }
     }
     catch (\Throwable $e) {

src/Element/ComponentPropForm.php

@@ -64,9 +64,38 @@ class ComponentPropForm extends ComponentFormBase {
         [$class, 'preRenderPropOrSlot'],
       ],
       '#theme_wrappers' => [],
+      '#after_build' => [
+              [$class, 'afterBuild'],
+      ],
     ];
   }
 
+  /**
+   * Alter the element after the form is built.
+   *
+   * @param array $element
+   *   The element.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The form state.
+   *
+   * @return array
+   *   The altered element.
+   *
+   * @SuppressWarnings(PHPMD.UnusedFormalParameter)
+   */
+  public static function afterBuild(array $element, FormStateInterface $form_state) : array {
+    if ($form_state->isProcessingInput()) {
+      // For browser-submitted forms, the submitted values do not contain
+      // values for certain elements (empty multiple select, unchecked
+      // checkbox). Child elements are processed after the parent element,
+      // The processed values, stored in the form_state, are propagated
+      // to the '#value' of the element here (parents will process).
+      $element['#value'] = $form_state->getValue($element['#parents']);
+      // Values may need to be cleaned if the form evolves.
+    }
+    return $element;
+  }
+
   /**
    * Build props forms.
    */

src/Element/ComponentSlotForm.php

@@ -80,9 +80,60 @@ class ComponentSlotForm extends ComponentFormBase {
       ],
       "#wrap" => TRUE,
       "#title_in_component" => NULL,
+      '#after_build' => [
+        [$class, 'afterBuild'],
+      ],
     ];
   }
 
+  /**
+   * Alter the element after the form is built.
+   *
+   * @param array $element
+   *   The element.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The form state.
+   *
+   * @return array
+   *   The altered element.
+   *
+   * @SuppressWarnings(PHPMD.UnusedFormalParameter)
+   */
+  public static function afterBuild(array $element, FormStateInterface $form_state) : array {
+    if ($form_state->isProcessingInput()) {
+      // For browser-submitted forms, the submitted values do not contain
+      // values for certain elements (empty multiple select, unchecked
+      // checkbox). Child elements are processed after the parent element,
+      // The processed values, stored in the form_state, are propagated
+      // to the '#value' of the element here (parents will process).
+      $element['#value'] = $form_state->getValue($element['#parents']);
+      // Values are cleaned
+      // similarly to $form_state->cleanValues();
+      self::cleanValues($element['#value']);
+    }
+    return $element;
+  }
+
+  /**
+   * Clean values.
+   *
+   * @param array $value
+   *   Value to clean.
+   */
+  private static function cleanValues(array &$value) : void {
+    if (isset($value['add_more_button'])) {
+      unset($value['add_more_button']);
+    }
+    if (isset($value['sources'])) {
+      foreach (Element::children($value['sources']) as $delta) {
+        $source = &$value['sources'][$delta];
+        if (isset($source['_remove'])) {
+          unset($source['_remove']);
+        }
+      }
+    }
+  }
+
   /**
    * Handle the rebuild of the form and operations.
    */

src/Element/ComponentSlotsForm.php

@@ -53,9 +53,41 @@ class ComponentSlotsForm extends ComponentFormBase {
       '#process' => [
         [$class, 'buildForm'],
       ],
+      '#after_build' => [
+        [$class, 'afterBuild'],
+      ],
     ];
   }
 
+  /**
+   * Alter the element after the form is built.
+   *
+   * @param array $element
+   *   The element.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The form state.
+   *
+   * @return array
+   *   The altered element.
+   *
+   * @SuppressWarnings(PHPMD.UnusedFormalParameter)
+   */
+  public static function afterBuild(array $element, FormStateInterface $form_state) : array {
+    if ($form_state->isProcessingInput() && isset($element['#value']) && is_array($element['#value'])) {
+      // For browser-submitted forms, the submitted values do not contain
+      // values for certain elements (empty multiple select, unchecked
+      // checkbox). Child elements are processed after the parent element,
+      // The processed values, stored in '#value', are bubbled up to the
+      // parent element here.
+      foreach ($element['#value'] as $slot => &$slot_value) {
+        if (isset($element[$slot]) && isset($element[$slot]['#value'])) {
+          $slot_value = $element[$slot]['#value'];
+        }
+      }
+    }
+    return $element;
+  }
+
   /**
    * Processes slots form element.
    *