Upsert is matching always on Id instead of by chossen value.

[PawelWozniak]

Describe the bug
Context. We have a list of domains and child object with monthly traffic values.
Master-Detail relation is used on Domain_Monthly_Traffic__c object.

The domain object has Domain Name field Text(255) (External ID) (Unique Case Insensitive)

To Reproduce
Steps to reproduce the behavior:

1. Choose insert operation, add attached csv file usage-report_2026-03 example.csv
2. In step 2b choose that you want to match by Domain_Name__c

4. Map fields, choose output folder, click finish
5. See error output "Domain: id value of incorrect type: pwtestaccount.com"

Expected behavior
It should match against selected field Domain_Name__c instead of Id.

Desktop (please complete the following information):

• OS: Windows 11
• Version 65.0.0