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
@@ -616,9 +616,7 @@
       // of processors if a matching base hook is found.
       foreach ($grouped_functions[$first_prefix] as $candidate) {
         if (preg_match("/^{$prefix}_preprocess_(((?:[^_]++|_(?!_))+)__.*)/", $candidate, $matches)) {
-          if (isset($cache[$matches[2]])) {
-            $processors[$candidate] = $matches[1];
-          }
+          $processors[$candidate] = $matches[1];
         }
       }
     }
@@ -640,10 +638,12 @@
         $base_hook = $hook;
         while (!isset($cache[$base_hook]) && $pos = strrpos($base_hook, '__')) {
           $base_hook = substr($base_hook, 0, $pos);
-          $cache[$base_hook]['preprocess functions'][] = $processor;
+          $cache[$hook] = $cache[$base_hook];
+          $cache[$hook]['base hook'] = $base_hook;
+          $cache[$hook]['preprocess functions'][] = $processor;
           // If the current hook is based on a pattern, get the base hook.
-          if (isset($cache[$base_hook]['base hook'])) {
-            $base_hook = $cache[$base_hook]['base hook'];
+          if (isset($cache[$hook]['base hook'])) {
+            $base_hook = $cache[$hook]['base hook'];
           }
         }
       }
diff -u b/core/modules/system/src/Tests/Theme/ThemeTest.php b/core/modules/system/src/Tests/Theme/ThemeTest.php
--- b/core/modules/system/src/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/src/Tests/Theme/ThemeTest.php
@@ -303,7 +303,16 @@
     drupal_theme_rebuild();
     for ($i = 0; $i < 2; $i++) {
       $this->drupalGet('theme-test/preprocess-suggestions');
-      $this->assertText('Theme hook implementor=test_theme_preprocess_theme_test_preprocess_suggestions__suggestion().', 'Theme hook ran with data available from a preprocess function for the suggested hook.');
+      $items = $this->CssSelect('.suggestion');
+      $expected_values = [
+        'Suggestion',
+        'Kitten',
+        'Kitten',
+        'Flamingo',
+      ];
+      foreach ($items as $key => $item) {
+        $this->assertEqual((string) $item, $expected_values[$key]);
+      }
     }
   }
 }
diff -u b/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php b/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php
--- b/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php
+++ b/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php
@@ -148,7 +148,17 @@
    * suggestions.
    */
   function preprocessSuggestions() {
-    return array('#theme' => 'theme_test_preprocess_suggestions__suggestion');
+    return [
+      [
+        '#theme' => 'theme_test_preprocess_suggestions',
+        '#foo' => 'suggestion',
+      ],
+      [
+        '#theme' => 'theme_test_preprocess_suggestions',
+        '#foo' => 'kitten',
+      ],
+      ['#theme' => 'theme_test_preprocess_suggestions__kitten__flamingo'],
+    ];
   }
 
 }
diff -u b/core/modules/system/tests/modules/theme_test/templates/theme-test-preprocess-suggestions.html.twig b/core/modules/system/tests/modules/theme_test/templates/theme-test-preprocess-suggestions.html.twig
--- b/core/modules/system/tests/modules/theme_test/templates/theme-test-preprocess-suggestions.html.twig
+++ b/core/modules/system/tests/modules/theme_test/templates/theme-test-preprocess-suggestions.html.twig
@@ -1 +1,4 @@
-{{ foo }}
+<div class="suggestion">{{ foo }}</div>
+{% if bar %}
+  <div class="suggestion">{{ bar }}</div>
+{% endif %}
diff -u b/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
--- b/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -56,7 +56,10 @@
     'function' => 'test_theme_not_existing_function',
   );
   $items['theme_test_preprocess_suggestions'] = array(
-    'variables' => array('foo' => ''),
+    'variables' => [
+      'foo' => '',
+      'bar' => '',
+    ],
   );
   return $items;
 }
@@ -96,14 +99,14 @@
  * Implements hook_theme_suggestions_HOOK().
  */
 function theme_test_theme_suggestions_theme_test_preprocess_suggestions($variables) {
-  return array('theme_test_preprocess_suggestions__' . 'suggestion');
+  return ['theme_test_preprocess_suggestions__' . $variables['foo']];
 }
 
 /**
  * Implements hook_preprocess_HOOK().
  */
 function theme_test_preprocess_theme_test_preprocess_suggestions(&$variables) {
-  $variables['foo'] =  'Theme hook implementor=theme_theme_test_preprocess_suggestions().';
+  $variables['foo'] = 'Theme hook implementor=theme_theme_test_preprocess_suggestions().';
 }
 
 /**
diff -u b/core/modules/system/tests/themes/test_theme/test_theme.theme b/core/modules/system/tests/themes/test_theme/test_theme.theme
--- b/core/modules/system/tests/themes/test_theme/test_theme.theme
+++ b/core/modules/system/tests/themes/test_theme/test_theme.theme
@@ -108,6 +108,8 @@
 
 /**
  * Tests a theme overriding a default hook with a suggestion.
+ *
+ * Implements hook_preprocess_HOOK().
  */
 function test_theme_preprocess_theme_test_preprocess_suggestions(&$variables) {
   $variables['foo'] = 'Theme hook implementor=test_theme_preprocess_theme_test_preprocess_suggestions().';
@@ -118,5 +120,19 @@
  */
 function test_theme_preprocess_theme_test_preprocess_suggestions__suggestion(&$variables) {
-  $variables['foo'] = 'Theme hook implementor=test_theme_preprocess_theme_test_preprocess_suggestions__suggestion().';
+  $variables['foo'] = 'Suggestion';
+}
+
+/**
+ * Tests a theme overriding a default hook with a suggestion.
+ */
+function test_theme_preprocess_theme_test_preprocess_suggestions__kitten(&$variables) {
+  $variables['foo'] = 'Kitten';
+}
+
+/**
+ * Tests a theme overriding a default hook with a suggestion.
+ */
+function test_theme_preprocess_theme_test_preprocess_suggestions__kitten__flamingo(&$variables) {
+  $variables['bar'] = 'Flamingo';
 }
 
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Theme/ThemeManager.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManager.php
@@ -286,12 +286,10 @@ protected function theme($hook, $variables = array()) {
           include_once $this->root . '/' . $include_file;
         }
       }
-      // Replace the preprocess functions with those from the base hook.
       if (isset($base_hook_info['preprocess functions'])) {
         // 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'] = $base_hook_info['preprocess functions'];
       }
     }
     if (isset($info['preprocess functions'])) {
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/templates/theme-test-preprocess-suggestions--suggestion.html.twig
@@ -0,0 +1 @@
+<div class="suggestion">{{ foo }}</div>