After hours of debugging the complete vendor/symfony/form/Form I was only able to find out that the FormEvents::SUBMIT converts my null value to Object.

This is my code.

$form = $this->createForm(PersonType::class, new Person());
$form->submit($request->request->all(), false);

if ($form->isSubmitted() && $form->isValid())
{
    $em->persist($entity);
    $em->flush();

    return $entity;
}

return $form->getErrors(true, true);

Then I send this request.

POST /api/persons/e3d90966-b2e7-4503-959f-da989c73c185
{
    "name": {"firstName": "John", "lastName": "Doe"}
}

It sets the name of the person to John Doe.

Now I want to clear it:

POST /api/persons/e3d90966-b2e7-4503-959f-da989c73c185
{
    "name": null
}

And et voilà: The name is still John Doe.

I'm using a custom type:

class NameType extends AbstractType
{
    private EntityManagerInterface $manager;

    public function __construct(EntityManagerInterface $manager)
    {
        $this->manager = $manager;
    }

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        parent::buildForm($builder, $options);
        $builder
            ->add('firstName', TextType::class)
            ->add('lastName', TextType::class)
        ;

        $builder->addModelTransformer(new NameTransformer($this->manager));
    }

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

And is used via:

$builder
    ->add('name', NameType::class)
;

Some more informations: