With pymongo 3.6, getMore requests do not explicitly set a readPreference. With version 3.9, getMore specifically sets readPreference to primary even when the connection sets secondary or secondaryPreferred. I didn't specifically test versions greater than 3.6 and less than 3.9, so I am not sure when this change was introduced.

Snippet to reproduce:

import pymongo

client = pymongo.MongoClient(port=27017, readPreference="secondary")
database = "db"
collection_name = "has_more_than_101_documents"
collection = client.db.collection_name

docs = list(collection.find({}).limit(102))
print(len(docs))

With 3.6, the getMore contains no readPreference setting.

With 3.9, the getMore contains $readPreference mode:primary, even when the initial cursor set secondary.

Consider changing this behavior such that the getMore requests respect the readPreference set by the cursor.