GenericReferenceField doesn't validate choices on upsert

Hi, I am trying to use the GenericReferenceField with more than one reference in the choices parameter and use the upsert_one() to update or create a new entry. Below are the example models for understanding.

class A(me.Document):
name = me.StringField()
meta = {"strict": False}

class B(me.Document):
name = me.StringField()
meta = {"strict": False}

class Z(me.Document):
name = me.StringField()
meta = {"strict": False}

class C(me.DynamicDocument):
category = me.StringField()
object_oid = me.GenericReferenceField(choices=[A, B])
meta = {"strict": True}

Using given models, below code is expected to create a new field if it doesn't exist because the object_oid we save is reference to model A and B as defined in the choices:
a_obj = A.objects.get(name='Apple')
C.objects(object_oid=a_obj.id).upsert_one({'category': 'C', 'object_oid': a_obj})
b_obj = B.objects.get(name='Banana')
C.objects(object_oid=b_obj.id).upsert_one({'category': 'C', 'object_oid': b_obj})

However, the below code has two scenarios where we try to save and upsert entries. As Z is not in the choices of the GenericReferenceField, both of these scenarios should throw a Validation error. However, on calling save() it validates the choices but on calling upsert_one() it doesn't validate.
z_obj = Z.objects.get(name='Zebra')
i) C(category='Z', device_oid=z_obj).save() // Throws the Validation error
ii) C.objects(object_oid=z_obj.id).upsert_one({'category': 'C', 'object_oid': z_obj}) // Doesn't throw a Validation error

Please let me know if this is a bug or is there any other way to validate the GenericReferenceField choices on using upsert()?

Thanks!

Start with the supplied models and reproducer, comparing validation during save() with the upsert_one() path. Trace how GenericReferenceField choices are handled for upserts; done when an invalid Z reference raises a ValidationError while valid A and B references still create or update entries.