diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 6fd9ce5..0c1c25e 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -605,54 +605,59 @@ protected function processExtension(array &$cache, $name, $type, $theme, $path)
    */
   protected function completeSuggestion($hook, array &$cache) {
     $previous_hook = $hook;
-    $incomplete_previous_hook = array();
-    while ((!isset($cache[$previous_hook]) || isset($cache[$previous_hook]['incomplete preprocess functions']))
-      && $pos = strrpos($previous_hook, '__')) {
-      if (isset($cache[$previous_hook]) && !$incomplete_previous_hook && isset($cache[$previous_hook]['incomplete preprocess functions'])) {
-        $incomplete_previous_hook = $cache[$previous_hook];
-        unset($incomplete_previous_hook['incomplete preprocess functions']);
+    $first_valid_hook = '';
+    // Continue looping if the candidate hook doesn't exist or if the candidate
+    // hook has incomplete preprocess functions, and if the candidate hook is a
+    // suggestion (has a double underscore).
+    while ((!isset($cache[$previous_hook]) || isset($cache[$previous_hook]['incomplete preprocess functions'])) && $pos = strrpos($previous_hook, '__')) {
+      // Find the first existing candidate hook that has incomplete preprocess
+      // functions.
+      if (isset($cache[$previous_hook]) && !$first_valid_hook && isset($cache[$previous_hook]['incomplete preprocess functions'])) {
+        $first_valid_hook = $previous_hook;
       }
       $previous_hook = substr($previous_hook, 0, $pos);
+      $this->mergePreprocessFunctions($hook, $previous_hook, $first_valid_hook, $cache);
+    }
 
-      // If base hook exists clone of it for the preprocess function
-      // without a template.
-      // @see https://www.drupal.org/node/2457295
-      if (isset($cache[$previous_hook]) && !isset($cache[$previous_hook]['incomplete preprocess functions'])) {
-        $cache[$hook] = $incomplete_previous_hook + $cache[$previous_hook];
-        if (isset($incomplete_previous_hook['preprocess functions'])) {
-          $diff = array_diff($incomplete_previous_hook['preprocess functions'], $cache[$previous_hook]['preprocess functions']);
-          $cache[$hook]['preprocess functions'] = array_merge($cache[$previous_hook]['preprocess functions'], $diff);
-        }
-        // If a base hook isn't set, this is the actual base hook.
-        if (!isset($cache[$previous_hook]['base hook'])) {
-          $cache[$hook]['base hook'] = $previous_hook;
-        }
-      }
+    // In addition to processing suggestions, include base hooks.
+    if (isset($cache[$hook]['base hook']) && isset($cache[$cache[$hook]['base hook']])) {
+      // Pass the current hook as the first valid hook if a base hook exists.
+      $this->mergePreprocessFunctions($hook, $cache[$hook]['base hook'], $hook, $cache);
     }
+
+    // Remove the incompleteness flag.
+    unset($cache[$hook]['incomplete preprocess functions']);
   }
 
   /**
-   * Merges the preceding hook's preprocess functions into the current hook's.
+   * Merges the source hook's preprocess functions into the destination hook's.
    *
-   * @param string $hook
+   * @param string $destination_hook_name
    *   The name of the suggestion hook to complete.
-   * @param string $complete_hook
+   * @param string $source_hook_name
    *   The name of the hook to merge preprocess functions from.
+   * @param string $parent_hook_name
+   *   The name of the parent hook if it exists.
    * @param array $cache
    *   The theme registry, as documented in
    *   \Drupal\Core\Theme\Registry::processExtension().
    */
-  protected function mergePreprocessFunctions($hook, $complete_hook, array &$cache) {
-    // A suggestion might be defined in a hook_theme implementation and
-    // include preprocess functions not following the pattern so we merge.
-    if (isset($cache[$hook]['preprocess functions'])) {
-      $preprocess_functions = array_merge($cache[$complete_hook]['preprocess functions'], $cache[$hook]['preprocess functions']);
+  protected function mergePreprocessFunctions($destination_hook_name, $source_hook_name, $parent_hook_name, array &$cache) {
+    $parent_hook = isset($cache[$parent_hook_name]['incomplete preprocess functions']) ? $cache[$parent_hook_name] : [];
+    // 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'])) {
+      $cache[$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']);
+        $cache[$destination_hook_name]['preprocess functions'] = array_merge($cache[$source_hook_name]['preprocess functions'], $diff);
+      }
+      // If a base hook isn't set, this is the actual base hook.
+      if (!isset($cache[$source_hook_name]['base hook'])) {
+        $cache[$destination_hook_name]['base hook'] = $source_hook_name;
+      }
     }
-    else {
-      $preprocess_functions = $cache[$complete_hook]['preprocess functions'];
-    }
-
-    $cache[$hook]['preprocess functions'] = $preprocess_functions;
   }
 
   /**
@@ -734,15 +739,7 @@ protected function postProcessExtension(array &$cache, ActiveTheme $theme) {
       // from a pattern. This is typically set from
       // drupal_find_theme_functions() and drupal_find_theme_templates().
       if (isset($info['incomplete preprocess functions'])) {
-        // Suggestion hooks inherit from their preceding suggestion hooks first
-        // whereas hooks with a manually set "base hook" key can just be
-        // completed based on the hook given.
-        if (strpos($hook, '__')) {
-          $this->completeSuggestion($hook, $cache);
-        }
-        else {
-          $this->mergePreprocessFunctions($hook, $info['base hook'], $cache);
-        }
+        $this->completeSuggestion($hook, $cache);
         unset($cache[$hook]['incomplete preprocess functions']);
       }
 
diff --git a/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
index c57ff91..1454311 100644
--- a/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
+++ b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
@@ -176,6 +176,8 @@ public function testGetRegistryForModule() {
 
   /**
    * @covers ::postProcessExtension
+   * @covers ::completeSuggestion
+   * @covers ::mergePreprocessFunctions
    *
    * @dataProvider providerTestPostProcessExtension
    *
@@ -203,7 +205,7 @@ public function testPostProcessExtension($defined_functions, $hooks, $expected)
     $reflection_method->setAccessible(TRUE);
     $reflection_method->invokeArgs($this->registry, [&$hooks, $theme->reveal()]);
 
-    $this->assertArrayEquals($hooks, $expected);
+    $this->assertArrayEquals($expected, $hooks);
   }
 
   /**
@@ -233,6 +235,28 @@ public function providerTestPostProcessExtension() {
         ],
       ],
     ];
+    $data['missing_base_hook'] = [
+      'defined_functions' => ['cat', 'mouse', 'cheese'],
+      'hooks' => [
+        'test_hook' => [
+          'preprocess functions' => ['cat', 'mouse'],
+        ],
+        'test_hook_manual' => [
+          'preprocess functions' => ['cheese'],
+          'base hook' => 'missing_hook',
+          'incomplete preprocess functions' => TRUE,
+        ],
+      ],
+      'expected' => [
+        'test_hook' => [
+          'preprocess functions' => ['cat', 'mouse'],
+        ],
+        'test_hook_manual' => [
+          'preprocess functions' => ['cheese'],
+          'base hook' => 'missing_hook',
+        ],
+      ],
+    ];
     $data['manual_suggestion'] = [
       'defined_functions' => [
         'cat',
@@ -241,8 +265,12 @@ public function providerTestPostProcessExtension() {
         'bread',
         'test_preprocess_test_hook__suggestion',
         'test_preprocess_test_hook__suggestion__another',
+        'and_now_for_something_completely_different',
       ],
       'hooks' => [
+        'unrelated_hook' => [
+          'preprocess functions' => ['and_now_for_something_completely_different'],
+        ],
         'test_hook' => [
           'preprocess functions' => ['cat', 'mouse'],
           'template' => 'test-hook',
@@ -255,8 +283,16 @@ public function providerTestPostProcessExtension() {
           'preprocess functions' => ['cheese'],
           'incomplete preprocess functions' => TRUE,
         ],
+        'test_hook__suggestion__with_base_hook' => [
+          'base hook' => 'unrelated_hook',
+          'preprocess functions' => ['something_new'],
+          'incomplete preprocess functions' => TRUE,
+        ],
       ],
       'expected' => [
+        'unrelated_hook' => [
+          'preprocess functions' => ['and_now_for_something_completely_different'],
+        ],
         'test_hook' => [
           'preprocess functions' => [
             'cat',
@@ -286,6 +322,18 @@ public function providerTestPostProcessExtension() {
           'base hook' => 'test_hook',
           'template' => 'test-hook',
         ],
+        'test_hook__suggestion__with_base_hook' => [
+          'preprocess functions' => [
+            'and_now_for_something_completely_different',
+            'cat',
+            'mouse',
+            'bread',
+            'test_preprocess_test_hook__suggestion',
+            'something_new',
+          ],
+          'base hook' => 'unrelated_hook',
+          'template' => 'test-hook',
+        ],
       ],
     ];
     return $data;