Bulk API import lead source most recent

Summary:

Unable to update the "Lead Source Most Recent" field on certain Eloqua contacts via the Bulk Import API when the field already contains a value.

Content (please ensure you mask any confidential information):

We create the following bulk import definition :

{    "name": "HUR_ImportContactsLeadSource",

    "fields": {        "ContactID": "{{Contact.Id}}", 

      "LeadSourceMostRecent": "{{Contact.Field(C_Lead_Source___Most_Recent1)}}" 

  }, 

  "identifierFieldName": "ContactID", 

  "isSyncTriggeredOnImport": true, 

  "updateRule": "always"

}

When we submit the request :

[    {        "ContactID": 1225674,   

    "LeadSourceMostRecent": "Qualtrics-subscribe"    }

]

Result :

        {            "syncUri": "/syncs/5225250",       

    "count": 1,            "severity": "information",            "statusCode": "ELQ-00022",   

        "message": "Contacts updated.",            "createdAt": "2026-03-17T15:45:49.7600000Z"        }

The result seems correct, but the contact is not updated. The field only updates if we manually clear the "lead source most recent" value.