Error multiple insert Symfony5 with JSON field

I have a problem with EntityType Multiple a register in database a result is : [{}, {}] because type is json but this json is empty... I don't fond a solve, can you help me ?

Controller:

public function gestion_group(Request $request, EntityManagerInterface $manager)
    {
        $groups = new GroupsManager();
        $form = $this->createForm(AdminGroupType::class , $groups);
        $form->handleRequest($request);

        if ($form->isSubmitted() && $form->isValid()) {
            $groups->getU();
            $manager->persist($groups);
            $manager->flush();
            $this->addFlash(
                'success',
                "La personne à bien été ajouter au groupe !"
            );

            return $this->redirectToRoute('admin');
        }
        return $this->render('admin/groups/index.html.twig', [
            'form' => $form->createView()
        ]);
    }

entity :

class GroupsManager
{
    /**
     * @ORM\Id()
     * @ORM\GeneratedValue()
     * @ORM\Column(type="integer")
     */
    private $id;

    /**
     * @ORM\ManyToOne(targetEntity="App\Entity\User", inversedBy="groupsManagers")
     */
    private $manager;

    /**
     * @ORM\Column(type="json")
     */
    private $u = [];

    public function getId(): ?int
    {
        return $this->id;
    }

    public function getManager(): ?User
    {
        return $this->manager;
    }

    public function setManager(?User $manager): self
    {
        $this->manager = $manager;

        return $this;
    }

    public function getU(): ?array
    {
        return $this->u;
    }

    public function setU(array $u): self
    {
        $this->u = $u;

        return $this;
    }
}

And form :

public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('manager', EntityType::class, array(
                'label' => 'Choisir le manager',
                'class' => User::class,
                'choice_label' => function ($user) {
                    return $user->getLastname() . '' . $user->getFirstname();
                }
            ))
            ->add('u', EntityType::class, array(
                'label' => 'Choisir un utilisateur',
                'class' => User::class,
                'choice_label' => function ($user) {
                    return $user->getLastname() . '' . $user->getFirstname();
                },
                'multiple' => true
            ));
    }

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

I would like add user with multiple choice for 1 manager to manage group of user i choice json type for it but is empty, can you help ?