symfony: [Serializer] Groups not working

Symfony version(s) affected: 4.4.x

Description
Hi,

I’m using groups to serialize data but it’s not always working.

For example :

    /**
     * @ORM\Column(type="array", nullable=true)
     * @Groups({"me_user", "write_user"})
     */
    protected $register_purposes;

        //return serialized data
        $serializedEntity = $this->container->get('serializer')->serialize($users, 'json', array('groups' => ['me_user']));
        return new Response($serializedEntity, 200, ['Content-Type' => 'application/json']);

Won’t work until I passed the field with public access. So : public $register_purposes;

Any idea why ?