diff -u b/core/lib/Drupal/Core/Theme/ThemeManager.php b/core/lib/Drupal/Core/Theme/ThemeManager.php
--- b/core/lib/Drupal/Core/Theme/ThemeManager.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManager.php
@@ -290,6 +290,7 @@
         // Set a variable for the 'theme_hook_suggestion'. This is used to
         // maintain backwards compatibility with template engines.
         $theme_hook_suggestion = $hook;
+        $info['preprocess functions'] = array_merge($base_hook_info['preprocess functions'], $info['preprocess functions']);
       }
     }
     if (isset($info['preprocess functions'])) {