Shipping Webhook does not work as documented

In the documentation, Webhooks: shipping – Snipcart Documentation, each entry in the items array has a property, “customFieldsJson” which is a JSON string. In the body received from the webhook however, this field is not present. A “customFields” property is present but is not a string, rather an empty array. It remains an empty array regardless of the presence of custom fields on the product, making it useless.

How can the handler for the shipping webhook access custom field selections?