Upsert Does Not Set ExternalId

When calling upsert(record, extId_Field), the extIdField is not set.

Below is current example. Creating a new record of a custom object, with External ID field EXTID__c.

The record gets created, but the external id field (EXTID__c) is not set.

const newProject = await sfconn.sobject("Project__c").upsert({
            Name: "EXAMPLE",
            RecordTypeId: recordTypeId,
            Account__c: "0016A00000EXAMPLE",
            Lead__c: "0056A00000EXAMPLE",
            EXTID__c: 123456
        }, "EXTID__c")

Start at the SObject.upsert(record, extId_Field) entry point and reproduce the custom-object example with EXTID__c. Trace the create path to determine why the external ID is not retained, then verify that a newly created record contains the supplied value and that existing upsert behavior still works.