Same problem that has been fixed here #22265
and here api-platform/core#1037

ObjectNormalizer returns $id instead of $Id. It is bad naming convention, but is possible

class Entity {
    protected $Id;

    public function getId()
    {
        return $this->Id;
    }
}

https://github.com/symfony/symfony/blob/v2.8.19/src/Symfony/Component/Serializer/Normalizer/ObjectNormalizer.php#L211

Is it valid, should I try to fix it?