Symfony 7中Flash Message无法正常显示问题求助

我的代码如下：

$form = $forumService->handleDiscussionForm($request);

if ($form === "submitted") {
    $this->addFlash('success', 'Your discussion has been successfully created and must be validated by Parentela.');
    return $this->redirectToRoute('app_forum_thematique', ['slug' => $thematique->getSlug()]);
}

测试1：提交表单后，页面正确跳转到app_forum_thematique路由，但Flash Message完全不显示。

测试2：修改代码后：

$form = $forumService->handleDiscussionForm($request);

if ($form === "submitted") {
    $this->addFlash('success', 'Your discussion has been successfully created and must be validated by Parentela.');
    dd("test");
    return $this->redirectToRoute('app_forum_thematique', ['slug' => $thematique->getSlug()]);
}

提交表单后页面显示test；此时移除dd()代码并刷新页面，Flash Message能正常显示，跳转也没问题。

为什么不加dd()的时候Flash Message就无法正常显示？有没有人遇到过同样的问题？

内容的提问来源于stack exchange，提问作者Alan