Setting primary key messes with the null kwarg

Mongoengine 0.20.0, PyMongo 3.10.1, Python 3.8.2, Windows 10, MongoDB Atlas
Specifying a field as being a primary key seems to mess with the null kwarg of other fields.

import mongoengine

mongoengine.connect(
	...
)

class ChannelBase(mongoengine.Document):
	test = mongoengine.IntField(null=True)
	meta = {
		"collection":"channels",
		"abstract":True
	}

class Channel1(ChannelBase):
	channelID = mongoengine.StringField(db_field="_id",required=True)
class Channel2(ChannelBase):
	channelID = mongoengine.StringField(primary_key=True,required=True)

ch1 = Channel1()
ch1.channelID = "test1"
ch1.save()

ch2 = Channel2()
ch2.channelID = "test2"
ch2.save()

The Channel1 document saves the test field as null, while the Channel2 document does not save it.

If this is intended, I cannot see why based off of the documentation.
This may also be an issue with MongoDB Atlas, but I highly doubt it.

The issue names no source files or tests. Start by running the provided Channel1 and Channel2 reproduction with the stated MongoEngine and MongoDB versions, then inspect the save and field-serialization path; done means the expected handling of the null field with a primary key is established and covered by a regression test.