diff --git a/core/lib/Drupal/Core/Config/StorableConfigBase.php b/core/lib/Drupal/Core/Config/StorableConfigBase.php
index a6c3287..6f20055 100644
--- a/core/lib/Drupal/Core/Config/StorableConfigBase.php
+++ b/core/lib/Drupal/Core/Config/StorableConfigBase.php
@@ -8,6 +8,7 @@
 namespace Drupal\Core\Config;
 
 use Drupal\Core\Config\Schema\Ignore;
+use Drupal\Core\Config\Schema\Mapping;
 use Drupal\Core\TypedData\PrimitiveInterface;
 use Drupal\Core\TypedData\Type\FloatInterface;
 use Drupal\Core\TypedData\Type\IntegerInterface;
@@ -215,6 +216,11 @@ protected function castValue($key, $value) {
       foreach ($value as $nested_value_key => $nested_value) {
         $value[$nested_value_key] = $this->castValue($key . '.' . $nested_value_key, $nested_value);
       }
+      if ($element instanceof Mapping && isset($element->getDataDefinition()['mapping'])) {
+        $mapping = $element->getDataDefinition()['mapping'];
+        $filled_keys = array_intersect_key($mapping, $value);
+        $value = array_merge($filled_keys, array_intersect_key($value, $filled_keys));
+      }
     }
     return $value;
   }