Laravel Echo & Pusher status code 200 but frontend no notification

I’m trying to broadcast notifications via a private channel to my web application for users who are logged in. I set up Laravel Echo and Pusher. When I test with php artisan tinker, I can see data in the debug console of Pusher-JS. I checked Telescope, and the status for [post] /broadcasting/auth is 200. However, despite all this, the frontend doesn’t display the notifications. Any suggestions on how to troubleshoot this issue would be greatly appreciated!

I ensure that App\Providers\BroadcastServiceProvider::class is not commented.

(Laravel 11) .env: BROADCAST_CONNECTION=pusher

AlertMenu.tsx:

   window.Pusher = Pusher;

    const echo = new Echo({
        broadcaster: 'pusher',
        key: "xxxxxxxxxxxxxxx",
        cluster: "mt1",
        forceTLS: true,
    });

AlertEvent.php: new PrivateChannel('project.'.$project_id.'.alerts');

BroadcastServiceProvider.php:

public function boot(): void
{
    Broadcast::routes();

    require base_path('routes/channels.php');
}

channels.php:

Broadcast::channel('project.{project_id}.alerts', function ($user, $project_id) {

    if($user->current_project_id == $project_id){
         return true;
     }else{
         return false;
   }
}