Symfony 6 - Fail to inject data into a dynamic formtype on POST_SUBMIT

I have a problem with a nested form. I can't get the values I want to pass. Here is a simple example to reproduce my problem, I would like to pre-fill a form about a user according to the selected house in my form.
Here are the files, if you want to test. I would like to inject the values of roger and billy the good way but my user fields are always empty

The models

class Test
{
    /**
     * @var string|null
     */
    private $house;

    /**
     * @var TestUser|null
     */
    private $user;

    // Getters & Setters of course...   
}

class TestUser
{
    /**
     * @var string|null
     */
    private $name;

    /**
     * @var int|null
     */
    private $age;

    // Getters & Setters again...
}

The main form

class TestType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options): void
    {
        $builder
            ->add('house', ChoiceType::class, [
                    'choices' => [
                        'first'  => 1,
                        'second' => 2,
                    ],
                ]
            );

        $builder->get('house')->addEventListener(FormEvents::POST_SUBMIT, [$this, 'addUser']);
    }


    function addUser(FormEvent $event)
    {
        $form  = $event->getForm()->getParent();
        $house = $event->getForm()->getData();

        if (!$house) {
            return;
        }

        // here is the part where I choose the user I want to use 
        // for the rest of the example (which does not work)
        $testUser = $house === 1
            ? (new TestUser())->setName('roger')->setAge(65)
            : (new TestUser())->setName('billy')->setAge(22);

        $builder = $form->getConfig()->getFormFactory()->createNamedBuilder('user', TestUserType::class, $testUser, [
            'auto_initialize' => false,
        ]);

        $form->add($builder->getForm());
    }

    public function configureOptions(OptionsResolver $resolver): void
    {
        $resolver->setDefaults([
            'data_class' => Test::class,
        ]);
    }
}

The user form type

class TestUserType extends AbstractType
{

    public function buildForm(FormBuilderInterface $builder, array $options): void
    {
        $builder
            ->add('name', TextType::class, ['label' => 'username'])
            ->add('age', IntegerType::class, ['label' => 'age']);
    }

    public function configureOptions(OptionsResolver $resolver): void
    {
        $resolver->setDefaults([
            'data_class' => TestUser::class,
        ]);
    }
}

The controller

public function testForm(Request $request): RedirectResponse|Response
{
    // ...
    $test = new Test();
    $form = $this->createForm(TestType::class, $test);
    $form->handleRequest($request);
    // ...
}

The view

{{ form_start(formtest) }}
  <p>
    <button class="btn btn-primary" type="submit">test go</button>
  </p>
{{ form_end(formtest) }}

all help is welcome