diff -u b/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
--- b/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -430,7 +430,7 @@
     return $conf;
   }
   // Keep the conf_path alive across drupal_static_reset() calls.
-  if (class_exists('Drupal\Component\Utility\Settings') && ($simpletest_conf_path = settings()->get('simpletest_conf_path'))) {
+  if (class_exists('Drupal\Component\Utility\Settings') && ($simpletest_conf_path = settings()->get('simpletest_conf_path')) && drupal_valid_test_ua()) {
     $conf = $simpletest_conf_path;
     return $conf;
   }