Binding a mocked instance in the app container doesn't work in parallel

I have a test that works in serial, but when I try to execute it in parallel it fails for some reason...

(The Operation class extends the Command class and is used for Crons).

class ImportEmailsTest extends TestCase
{
    public function testOperation(): void
    {
        $mockProcessor = Mockery::mock(MailboxProcessor::class)
            ->shouldReceive('importFromMailbox')->once()
            ->getMock();

        $this->app->instance(MailboxProcessor::class, $mockProcessor);

        $this->artisan('import-emails')
            ->assertSuccessful();
    }
}

class ImportEmails extends Operation
{
    protected $key = 'import-emails';

    public function __construct(
        protected MailboxProcessor $mailProcessor,
    ) {
        parent::__construct();
    }

    public function operation()
    {
        $this->mailProcessor->importFromMailbox();
    }
}

Any thoughts? I can't seems to figure out why it doesn't work. If I dd(app()->has(MailboxProcessor::class)); in the operation method it returns true.