diff -u b/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
--- b/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -138,22 +138,6 @@
   protected $themeManager;
 
   /**
-   * User function names grouped by the word before the first underscore.
-   *
-   * @var string[]
-   */
-  protected $groupedFunctions = [];
-
-  /**
-   * All user-defined functions that have been added to groupedFunctions.
-   *
-   * The array keys are the function names, for faster dereferencing.
-   *
-   * @var bool[]
-   */
-  protected $seenFunctions = [];
-
-  /**
    * Constructs a \Drupal\Core\Theme\Registry object.
    *
    * @param string $root
@@ -751,23 +735,14 @@
   public function getPrefixGroupedUserFunctions() {
     $functions = get_defined_functions();
 
+    $grouped_functions = [];
     // Splitting user defined functions into groups by the first prefix.
     foreach ($functions['user'] as $function) {
-      if (isset($this->seenFunctions[$function])) {
-        continue;
-      }
       list($first_prefix,) = explode('_', $function, 2);
-      $this->groupedFunctions[$first_prefix][] = $function;
-    }
-
-    // The theme registry may load new code. On encountering newly defined
-    // functions, we save the list of defined functions again. This works
-    // because functions cannot disappear between calls.
-    if (isset($first_prefix)) {
-      $this->seenFunctions = array_fill_keys($functions['user'], TRUE);
+      $grouped_functions[$first_prefix][] = $function;
     }
 
-    return $this->groupedFunctions;
+    return $grouped_functions;
   }
 
   /**
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Render/theme.api.php
+++ b/core/lib/Drupal/Core/Render/theme.api.php
@@ -99,10 +99,12 @@
  * before the template file is invoked to modify the variables that are passed
  * to the template. These make up the "preprocessing" phase, and are executed
  * (if they exist), in the following order (note that in the following list,
- * HOOK indicates the theme hook name, MODULE indicates a module name, THEME
- * indicates a theme name, and ENGINE indicates a theme engine name). Modules,
- * themes, and theme engines can provide these functions to modify how the
- * data is preprocessed, before it is passed to the theme template:
+ * HOOK indicates the hook being called or a less specific hook. For example, if
+ * '#theme' => 'node__article' is called, hook is node__article and node. MODULE
+ * indicates a module name, THEME indicates a theme name, and ENGINE indicates a
+ * theme engine name). Modules, themes, and theme engines can provide these
+ * functions to modify how the data is preprocessed, before it is passed to the
+ * theme template:
  * - template_preprocess(&$variables, $hook): Creates a default set of variables
  *   for all theme hooks with template implementations. Provided by Drupal Core.
  * - template_preprocess_HOOK(&$variables): Should be implemented by the module