diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index efa3446..a3054e3 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -647,7 +647,7 @@ protected function mergePreprocessFunctions($destination_hook_name, $source_hook
     // If base hook exists clone of it for the preprocess function
     // without a template.
     // @see https://www.drupal.org/node/2457295
-    if (isset($cache[$source_hook_name]) && !isset($cache[$source_hook_name]['incomplete preprocess functions'])) {
+    if (isset($cache[$source_hook_name]) && (!isset($cache[$source_hook_name]['incomplete preprocess functions']) || !isset($cache[$destination_hook_name]['incomplete preprocess functions']))) {
       $cache[$destination_hook_name] = $parent_hook + $cache[$source_hook_name];
       if (isset($parent_hook['preprocess functions'])) {
         $diff = array_diff($parent_hook['preprocess functions'], $cache[$source_hook_name]['preprocess functions']);
diff --git a/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
index e74a904..8be62d8 100644
--- a/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
+++ b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
@@ -363,7 +363,6 @@ public function providerTestPostProcessExtension() {
 
     // Same as above, but also test that such child hooks can also be extended
     // with magically named suggestions.
-    /*
     $data['child_hook_with_suggestions'] = [
       'defined_functions' => [
         'test_preprocess_child_hook__suggestion',
@@ -411,7 +410,6 @@ public function providerTestPostProcessExtension() {
         ],
       ],
     ];
-    */
 
     // Test that a suggestion following the "__" naming pattern can specify a
     // different base hook than what is implied by that pattern. Ensure that