SynchronizedField for a ParentalManyToManyField leads to TypeError: 'DeferringManyRelatedManager' object is not iterable

[hpoul]

Hello,
I have a translatable snippet with a ParentalManyToManyField to another model and want it to be synchronized. But when trying to translate this page I get the following exception:

Traceback (most recent call last):
  File "/Users/herbert/dev/python/wagtail-localize/wagtail_localize/tests/test_translation_model.py", line 721, in test_parental_many_to_many
    translation.save_target()
  File "/Users/herbert/dev/python/wagtail-localize/wagtail_localize/models.py", line 1340, in save_target
    self.source.create_or_update_translation(
  File "/Users/herbert/dev/python/wagtail-localize/wagtail_localize/models.py", line 762, in create_or_update_translation
    copy_synchronised_fields(original, translation)
  File "/Users/herbert/dev/python/wagtail-localize/wagtail_localize/fields.py", line 296, in copy_synchronised_fields
    setattr(target, field.attname, getattr(source, field.attname))
  File "/Users/herbert/dev/python/wagtail-localize/venv/lib/python3.9/site-packages/modelcluster/fields.py", line 490, in __set__
    manager.set(value)
  File "/Users/herbert/dev/python/wagtail-localize/venv/lib/python3.9/site-packages/modelcluster/fields.py", line 424, in set
    objs = list(objs)
TypeError: 'DeferringManyRelatedManager' object is not iterable

I have created a quick repro test here:

https://github.com/hpoul/wagtail-localize/blob/64766882587d02b64822e903a21842b3c829e38f/wagtail_localize/test/models.py#L61-L68

and

https://github.com/hpoul/wagtail-localize/blob/64766882587d02b64822e903a21842b3c829e38f/wagtail_localize/tests/test_translation_model.py#L708-L725

@register_snippet
class TestParentalSnippet(TranslatableMixin, models.Model):
    field = ParentalManyToManyField(
        TestUUIDModel,
        blank=True,
    )

    translatable_fields = [SynchronizedField("field")]

and

    def test_parental_many_to_many(self):
        foreign_key_target = TestUUIDModel.objects.create(charfield="Some Test")
        snippet = TestParentalSnippet.objects.create()
        snippet.field = [foreign_key_target]

        source, created = TranslationSource.get_or_create_from_instance(snippet)
        translation = Translation.objects.create(
            source=source,
            target_locale=self.fr_locale,
        )

        translation.save_target()

Am I doing something wrong? Should ParentalManyToManyField be supported to be a synchronized field?

thanks,
Herbert