I've tried:
customers?expand=addresses,emailAddresses,phonenumbers,cards,orders&return_null_fields=true
and can see the Customers with Order IDs. In the API docs for Orders, it says you can expand on 'customers' to return an array like so:
orders?expand=customers,payments&return_null_fields=true&filter=id=0J2MK6G4AY168
...although the response we get has no Customers array in it. I am targeting the order ID provided from the first Customers query, so I am sure this order should be linked to a customer.