diff -u b/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
--- b/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -125,6 +125,8 @@
       $all_profiles = $profiles_scanner ->scan('/^' . DRUPAL_PHP_FUNCTION_PATTERN . '\.profile$/', 'profiles');
       // If a module is within a profile directory but specifies another
       // profile for testing, it needs to be found it in the parent profile.
+      // config_get_config_directory() fires drupal_valid_test_ua() so this
+      // global is populated.
       if (!empty($GLOBALS['drupal_test_info']['original_config_directory_active'])) {
         $config_directory = $GLOBALS['drupal_test_info']['original_config_directory_active'];
         if (empty($config_directory['absolute'])) {
@@ -182,7 +184,7 @@
     if (!empty($GLOBALS['drupal_test_info']['test_run_id'])) {
       $parts[] = $GLOBALS['drupal_test_info']['test_run_id'];
     }
-    if ($prefix = drupal_valid_test_ua()) {
+    elseif ($prefix = drupal_valid_test_ua()) {
       $parts[] = $prefix;
     }
     return implode('_', $parts);