Cascading save of Reference ListField fails

When I have two models that are linked to each other via a One-To-Many list of references, a cascading save seems to not work.

Run the following code and observe in your database how User is saved, but Comment is not. In other scenarios I have seen this to fail with a ValidationError (because the reference cant be resolved), but in this example it seems to fail silently.

I have tried using the setting "cascade": True on the "meta" variable of the document instead, but the effect is the same.

from typing import List
from uuid import UUID, uuid4

from mongoengine import Document, UUIDField, ReferenceField, StringField, ListField, connect

connect(host='mongodb://localhost/docker')


class Comment(Document):
    id: UUID = UUIDField(primary_key=True, default=uuid4)
    text = StringField(required=True)


class User(Document):
    id: UUID = UUIDField(primary_key=True, default=uuid4)
    comments: List[Comment] = ListField(ReferenceField('Comment'))


comment1 = Comment(text="hello world")
comment2 = Comment(text="goodbye world")
user = User(comments=[comment1, comment2])

user.save(cascade=True)

Start by running the supplied Python example against MongoDB and inspect the save(cascade=True) behavior for User.comments, ListField, and ReferenceField. Done means the User and both Comment documents are persisted consistently, without the silent failure or unresolved-reference ValidationError described.