Recursively resolve custom fields

[apiening]

I have added a custom field to Virtual Machines targeting Contacts. With the option flatten_custom_fields: True in my inventory definition, the relation is resolved as expected.
However, the related Contacts do only resolve the fields display, id, name and url. I do understand that the URL does point to the resource with all fields. However, I need to have access to all fields of the assigned Contacts available in my inventory, including Custom fields.

Am I missing an option to fetch all related fields, or is this not possible with the netbox inventory?