`save` after `switch_collection` save only the document's id

I have a very strange behaviour when saving documents after using the switch_collection method. It saves the document on a new collection, but without the value of its fields, only with the _id field.

Can be reproduced using the following code:

import mongoengine as me


class ObjectA(me.Document):
    name = me.StringField()

def test_strange_behaviour():
    a_0 = ObjectA(name="a_0")
    a_0.save()

    a_1 = ObjectA(name="a_1")
    a_1.save()
    
    a_1.switch_collection("new_collection", keep_created=False)
    a_1.id = a_0.id
    a_1.save()

    print(a_1._collection)
    print(a_1._collection.find_one(a_1.id))

When calling the test_strange_behaviour function, the code prints something like that:

Collection(Database(MongoClient(host=['localhost:27017'], document_class=dict, tz_aware=False, connect=True, read_preference=Primary(), connecttimeoutms=30000, heartbeatfrequencyms=3000, ssl=False), 'engine'), 'new_collection')
{'_id': ObjectId('5dc57611dba04ca06410e477')}

As you can see, the last print shows that the document created on the new_collection collection contains only the _id field, and not the name field.

Start by running the provided test_strange_behaviour reproduction with MongoDB and inspect the switch_collection and save entry points. Confirm that saving after switching to new_collection preserves the document's name field, not only its _id.