upsert_one returns None even with primary read preference

It seems from the documentation that upsert_one should always return an object, but I keep getting back None on freshly created objects. E.g in pseudocode this always works in development, but fails in production:

new_object = ModelClass.objects(application=application_id).upsert_one(set_on_insert__date_created=now)
self.assertIsNotNone(new_object)

I assumed it might be related to the read preference in production (it is secondary), and tried the following:

new_object = ModelClass.                                    \
                objects(application=application_id).        \
                read_preference(Primary()).                 \
                upsert_one(set_on_insert__date_created=now)
self.assertIsNotNone(new_object)

Still doesn't work. The problem happens when line 564 gets triggered in queryset/base.py:

document = self._document.objects.with_id(atomic_update.upserted_id)

If - instead of self._document.objects.with_id - I re-do the query (and append the id), then it seems to take read preference into account, and finally works:

def _safe_upsert(app, **kwargs):
    def _query(**query_params):
        return ModelClass.                                     \
                    objects(**query_params).                   \
                    read_preference(read_pref.to_pymongo())

    query = _query(application=app.id)
    mlog.debug("Query class: %s", query.__class__)

    atomic_update = query.update(
                            multi=False,
                            upsert=True,
                            full_result=True,
                            **kwargs)
    if atomic_update.raw_result['updatedExisting']:
        return query.get()
    else:
        return _query(id=atomic_update.upserted_id)

Issue occurs with .no_cache() and using the default mongoengine QuerySet classes.

Start in queryset/base.py at line 564, where upsert_one retrieves the upserted document through self._document.objects.with_id, and compare that path with the re-created query shown in the report. Reproduce the None result with no_cache(), the default QuerySet classes, and primary read preference; done means freshly upserted objects are returned consistently.