Les groupes de validation Symfony 4 CollectionType ne fonctionnent pas

J'essaie de valider FormType à l'intérieur de CollectionType avec quelques règles de groupes simples mais cela ne fonctionne pas, mais si j'essaie de faire la même chose sans groupes de validations, cela fonctionne bien. Une idée? Ceci est un exemple complet et simple qui reproduit l'erreur https://github.com/ychakroun / symfony-collection-type-issue

class LinkType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('title', TextType::class, [
                'label' => 'Titre du menu:',
                'attr' => [
                    'input-group' => 'true',
                ],
            ])
            ->add('external', ChoiceType::class, [
                'label' => false,
                'expanded' => true,
                'choices' => [
                    'Lien interne' => false,
                    'Lien externe' => true,
                ],
                'choice_attr' => [
                    'class' => 'link-type',
                ],
                'label_attr' => [
                    'class' => 'btn-group btn-group-toggle',
                    'data-toggle' => 'buttons',
                ]
            ])
            ->add('url', UrlType::class, [
                'label' => 'SAISISSEZ L\'URL EXTERNE',
                'attr' => ['placeholder' => 'https://'],
            ])
            ->add('pageVersion', EntityType::class, [
                'required' => false,
                'class' => Page::class,
                'choice_label' => 'name',
            ])
            ->add('category', EntityType::class, [
                'required' => false,
                'class' => Category::class,
                'choice_label' => 'title',
                'query_builder' => function (CategoryRepository $er) {
                    return $er->createQueryBuilder('c')->where('c.enabled = 1');
                },
            ])
            ->add('position', HiddenType::class, [
                'attr' => [
                    'class' => 'my-position',
                ],
            ])
        ;
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults([
            'data_class' => Link::class,
            'cascade_validation' => true,
            'validation_groups' => function (FormInterface $form) {
                /** @var Link $link */
                $link = $form->getData();
                $groups = ['Default'];
                if ($link->getExternal()) {
                    $groups[] = 'isExternal';
                } else {
                    $groups[] = 'isInternal';
                }

                return $groups;
            },
        ]);
    }
}

class StickerType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('links', CollectionType::class, [
                'entry_type' => LinkType::class,
                'allow_add' => true,
                'allow_delete' => true,
                'delete_empty' => true,
                'attr' => [
                    'class' => 'collection',
                ],
                'by_reference' => false,
            ])
        ;
    }

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

Et voici les formulaires que j'utilise:

/**
 * Link
 *
 * @ORM\Table(name="link")
 * @ORM\Entity(repositoryClass="App\Repository\LinkRepository")
 */
    class Link
    {

    /**
     * @var mixed
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="IDENTITY")
     */
    private $id;

    /**
     * @var string|null
     * @Assert\NotBlank()
     *
     * @ORM\Column(name="title", type="string")
     */
    private $title;

    /**
     * @var bool
     *
     * @ORM\Column(name="external", type="boolean")
     */
    private $external;

    /**
     *
     * @var string|null
     *
     * @Assert\NotBlank(groups={"isExternal"})
     * @Assert\Url(groups={"isExternal"})
     * @ORM\Column(name="url", type="text", nullable=true)
     */
    private $url;

    /**
     * @var \App\Entity\PageVersion|null
     *
     * @Assert\NotBlank(groups={"isInternal"})
     * @ORM\ManyToOne(targetEntity="App\Entity\PageVersion")
     * @ORM\JoinColumns({
     *   @ORM\JoinColumn(name="page_version_id", referencedColumnName="id", nullable=true)
     * })
     */
    private $pageVersion;

    /**
     * @var \App\Entity\Category|null
     *
     * @Assert\NotBlank(groups={"isInternal"})
     * @ORM\ManyToOne(targetEntity="App\Entity\Category", inversedBy="links")
     * @ORM\JoinColumns({
     *   @ORM\JoinColumn(name="category_id", referencedColumnName="id", nullable=true)
     * })
     */
    private $category;

    /**
     * @var \App\Entity\Sticker|null
     *
     * @ORM\ManyToOne(targetEntity="App\Entity\Sticker", inversedBy="links")
     * @ORM\JoinColumns({
     *   @ORM\JoinColumn(name="sticker_id", referencedColumnName="id", nullable=true)
     * })
     */
    private $sticker;
}