diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index bb89707..cb9fe42 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -490,8 +490,11 @@ protected function processExtension(array &$cache, $name, $type, $theme, $path)
           $info['preprocess functions'] = array();
           $prefixes = array();
           if ($type == 'module') {
-            // Default variable preprocessor prefix.
-            $prefixes[] = 'template';
+            // Add template preprocessor only for templates.
+            if (!isset($info['function'])) {
+              // Default variable preprocessor prefix.
+              $prefixes[] = 'template';
+            }
             // Add all modules so they can intervene with their own variable
             // preprocessors. This allows them to provide variable preprocessors
             // even if they are not the owner of the current hook.
diff --git a/core/modules/system/src/Tests/Theme/RegistryTest.php b/core/modules/system/src/Tests/Theme/RegistryTest.php
index 8872f10..9960a2c 100644
--- a/core/modules/system/src/Tests/Theme/RegistryTest.php
+++ b/core/modules/system/src/Tests/Theme/RegistryTest.php
@@ -99,6 +99,11 @@ public function testMultipleSubThemes() {
       'template_preprocess',
       'test_basetheme_preprocess_theme_test_template_test',
     ], $preprocess_functions);
+
+    $preprocess_functions = $registry_base_theme->get()['theme_test_function_suggestions']['preprocess functions'];
+    $this->assertIdentical([
+      'test_basetheme_preprocess_theme_test_function_suggestions',
+    ], $preprocess_functions);
   }
 
   /**
diff --git a/core/modules/system/tests/themes/test_basetheme/test_basetheme.theme b/core/modules/system/tests/themes/test_basetheme/test_basetheme.theme
index 739439f..55ee468 100644
--- a/core/modules/system/tests/themes/test_basetheme/test_basetheme.theme
+++ b/core/modules/system/tests/themes/test_basetheme/test_basetheme.theme
@@ -27,3 +27,9 @@ function test_basetheme_views_post_render(ViewExecutable $view) {
  */
 function test_basetheme_preprocess_theme_test_template_test(&$variables) {
 }
+
+/**
+ * Implements hook_preprocess_HOOK() for theme_test_function_suggestions theme functions.
+ */
+function test_basetheme_preprocess_theme_test_function_suggestions(&$variables) {
+}