GenericEmbeddedDocumentField validation errors on .upsert but not on .save

Hi, I have a document defined with GenericEmbeddedDocumentField field;

class BrandItem(mg.Document):
    """

    BrandItems of a brand in the inventory

    """

    # inventory name of the brandItem
    name = StringField(required=True, unique_with='brand')

    # Brand under which the brandItems fall
    # If the brand is deleted this brand item will also be deleted
    brand = ReferenceField('Brand', reverse_delete_rule=CASCADE)

    # item type stores the type of items and item body stores the object of the type
    item_type = EnumField(ItemType, required=True)
    item_body = GenericEmbeddedDocumentField(required=True)

    # Auto update fields
    created = DateTimeField(default=datetime.utcnow())
    modified = DateTimeField()

    is_deleted = BooleanField(default=False)

When trying to save instances of this model I get different behaviours when creating an object and saving (works) and when trying to upsert it via the QuerySet manager (fails).

The error is

mongoengine.errors.ValidationError: Invalid embedded document instance provided to an GenericEmbeddedDocumentField

Interestingly, If I define the document with a DictField the behaviour swaps (upsert works and saves validation complains).

The upsert command:

    brand_item_dict = dict(
        name="Honey butter",
        brand=bid,
        is_excluded_from_gp_calculation=False,
        category_name="Condiments",
        item_type=ItemType.SUPPLIED_ITEM,
        item_body=SuppliedItem(
            purchasing_info=[
                    PurchasingInfo(
                        supplier="Nory Sup",
                        pack_size=5,
                        unit_of_measure=UnitOfMeasure.LITRE,
                        packs_per_case=10,
                        price=100,
                        currency=Currency.EURO,
                        price_unit_type=PriceUnitType.CASE,
                        is_main_purchasing_order_info=True,
                        supplier_code="item.supplier_code",
                )
            ]
        )

    )

    res = BrandItem.objects.upsert_one(
        **brand_item_dict
    )

The "same" command works when creating the brand object directly:

brand_item = BrandItem(
        name="Honey butter",
        brand=bid,
        is_excluded_from_gp_calculation=False,
        category_name="Condiments",
        item_type=ItemType.SUPPLIED_ITEM,
        item_body=SuppliedItem(
            purchasing_info=[
                    PurchasingInfo(
                        supplier="Nory Sup",
                        pack_size=5,
                        unit_of_measure=UnitOfMeasure.LITRE,
                        packs_per_case=10,
                        price=100,
                        currency=Currency.EURO,
                        price_unit_type=PriceUnitType.CASE,
                        is_main_purchasing_order_info=True,
                        supplier_code="item.supplier_code",
                )
            ]
        )

    )
    brand_item.save()

Start by reproducing the difference between QuerySet.upsert_one and Document.save with GenericEmbeddedDocumentField, using the BrandItem and SuppliedItem examples in the issue. Compare the validation behavior with DictField and determine what consistent validation result should be observed for both paths.