Issue with API: custom data in bulk uploads

I am updating data in the V3 api via bulk upload, but it is not updating the custom data.

Here is the URI I cam calling:

 

This is the body of the call
{"import_data":[{"email":"[email protected]","first_name":"Almost","last_name":"Absalom","company_name":"Easter","cf:dechexMemberID":"c3","cf:electionToken":"cdb52fa8e524257c"}],"list_ids":["ab81b330-ccc4-11ed-b245-fa163e25e9d8"]}

 

The call goes through fine, no errors are returned, contacts are added to the lists, but the account custom fields dechexMemberID & electionToken are not updated in the contact's records and the custom fields are blank

 in emails.

The developer guide says that all fields are updated in a bulk import (and this is how it worked in the v2 api).

Please advise!