Custom variable removed in the handle method

Its because when the $data is queued, laravel recognises that $data is actually a User model and so only queues the model ID. Later when the job is run, the model is fetched from the database, but subject is not a model property so it does not get restored.

One solution is to just pass them separately.

use App\Models\User;

class SendTestEmail implements ShouldQueue
{
    use Dispatchable, InteractsWithQueue, Queueable, SerializesModels;

    protected User $user;
    protected string $subject;

    /** 
     * Create a new job instance.
     *
     * @return void
     */
    public function __construct(User $user, string $subject)
    {   
        $this->user = $user;
		$this->subject = $subject;

        logger('__construct()'.$this->subject);
    }   

    /** 
     * Execute the job.
     *
     * @return void
     */
    public function handle()
    {   
        logger('handle(): '.$this->subject);
         Mail::to($user->email)->send(new AppMailer($user));
    }
}  

Email model:

    public static function sendTestEmail()
    {   
        SendTestEmail::dispatch(auth()->user(), 'Test email');
    }

ps, in PHP 8.1, you can simplify

    /** 
     * Create a new job instance.
     *
     * @return void
     */
    public function __construct (
            protected User $user,
            protected string $subject
    ) {   
        logger('__construct()'.$this->subject);
    }