diff --git a/core/modules/user/tests/src/Unit/UserAuthTest.php b/core/modules/user/tests/src/Unit/UserAuthTest.php
index 5794d14..899799f 100644
--- a/core/modules/user/tests/src/Unit/UserAuthTest.php
+++ b/core/modules/user/tests/src/Unit/UserAuthTest.php
@@ -280,4 +280,57 @@ class UserAuthTest extends UnitTestCase {
     $this->assertSame("$frontend_url?check_logged_in=1", $response->getTargetUrl());
   }
 
+  /**
+   * Tests the auth that ends in a redirect from subdomain with a fragment to TLD.
+   */
+  public function testAddCheckToUrlForTrustedRedirectResponseWithFragment(): void {
+    $site_domain = 'site.com';
+    $frontend_url = "https://$site_domain";
+    $backend_url = "https://api.$site_domain";
+    $request = Request::create($backend_url);
+    $response = new TrustedRedirectResponse($frontend_url . '#a_fragment');
+
+    $request_context = $this->createMock(RequestContext::class);
+    $request_context
+      ->method('getCompleteBaseUrl')
+      ->willReturn($backend_url);
+
+    $container = new ContainerBuilder();
+    $container->set('router.request_context', $request_context);
+    \Drupal::setContainer($container);
+
+    $session_mock = $this->createMock(SessionInterface::class);
+    $session_mock
+      ->expects($this->once())
+      ->method('has')
+      ->with('check_logged_in')
+      ->willReturn(TRUE);
+    $session_mock
+      ->expects($this->once())
+      ->method('remove')
+      ->with('check_logged_in');
+
+    $event_mock = $this->createMock(ResponseEvent::class);
+    $event_mock
+      ->expects($this->once())
+      ->method('getResponse')
+      ->willReturn($response);
+    $event_mock
+      ->expects($this->exactly(3))
+      ->method('getRequest')
+      ->willReturn($request);
+
+    $request
+      ->setSession($session_mock);
+
+    $this
+      ->getMockBuilder(Cookie::class)
+      ->disableOriginalConstructor()
+      ->onlyMethods([])
+      ->getMock()
+      ->addCheckToUrl($event_mock);
+
+    $this->assertSame("$frontend_url?check_logged_in=1#a_fragment", $response->getTargetUrl());
+  }
+
 }