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Yup, easy to work around. If you take a look at the API you'll see that they don't return custom fields with the bulk users api. You have to loop through and call with the ID for each user. Here's some code that can guide you.

test.py

import requests
import os
import time
from datetime import datetime

# Save your access token for RM in your environments variables.
ACCESS_TOKEN = os.environ.get('TOKEN')
BASEURL = "https://api.rm.smartsheet.com"
USERS_API = "/api/v1/users?per_page=100&page=1&fields=email"
CUSTOM_FIELDS_URL = "/api/v1/users/{}?fields=custom_field_values"


def test():
    # Getting the user list
    user_api_result = requests.get("{}{}".format(BASEURL, USERS_API),
                                headers={'Content-Type': 'application/json', 'auth': '{}'.format(ACCESS_TOKEN)})

    user_list = user_api_result.json()

    for user in user_list["data"]:
        # Sleeping here because the API has pretty restrictive limitations and I don't want to spam GChat
        time.sleep(1)
        custom_fields_url = CUSTOM_FIELDS_URL.format(user["id"])

        # Getting the custom fields for each user in turn
        custom_result = requests.get("{}{}".format(BASEURL, custom_fields_url),
                                     headers={'Content-Type': 'application/json',
                                              'auth': '{}'.format(ACCESS_TOKEN)})

        user_custom_fields = custom_result.json()

        print("User: {} has the following custom fields: {}".format(user["first_name"], user_custom_fields["custom_field_values"]["data"]))


if __name__ == '__main__':
    test()

You'll note that the custom fields data is a list, so you'll need to loop through that or squash it with a list comprehension or something.

Have fun! 😎