Document.update() updates mongodb but not python object for lists of embedded documents

I've run into a issue with updating a list of embedded documents. There is an easy way around it for now, but I think it qualifies as a bug since it doesn't do what you'd expect as a python developer. The following code will provide an explanation better than I could with just English:

from mongoengine import Document, EmbeddedDocument, StringField, \
                        ListField, EmbeddedDocumentField, connect
import mongomock


# create a document that contains a list of embedded documents
class Book(EmbeddedDocument):
    title = StringField(required=True)

class Library(Document):
    shelf = ListField(EmbeddedDocumentField(Book))

# connect to test DB
connect(db="test", host="mongomock://localhost")

# create a book and a library with that book in it
book = Book(title="My Book")
library = Library()
library["shelf"].append(book)
library.save()

# Querying the DB, we see that the book saved correctly
print(Library.objects().first()["shelf"])

# Remove the book from the library
library.update(pull__shelf=book)

# Querying the DB again, we can see that the embedded Book document was removed
print(f"Library.objects().first()['shelf'] --> {Library.objects().first()['shelf']}")

# But list of embedded documents in the python object we just updated remains unchanged
print(f"library['shelf'] --> {library['shelf']}")

Obviously it's not hard as a programmer to just query the DB again to get the updated list of embedded documents, but that isn't expected behavior from a python object perspective. Additionally, it creates an extra call to the database when the data could be retrieved locally.

Start with the provided Python reproduction around Document.update() and the pull__shelf operation, using mongomock to observe both database and object state. Trace how the database update is applied versus the in-memory list of embedded documents. Done means the embedded Book is removed from MongoDB and the updated Python object without requiring an extra reload.