SOAP API: Payload data (Opportunity fields) wiped out after successful createTransaction and updateT

Summary:

Hello CPQ Community,

I am working on an integration (via middleware/OIC) to automatically create CPQ transactions using the SOAP API.

Current Status (Success):
I have successfully implemented the "two-step" pattern:

createTransaction (returns the new internal ID).

updateTransaction (passing the ID and triggering our custom save action, e.g., cleanSave_t).
The transaction successfully moves out of the start_step, enters the workflow, and is fully visible in the Transaction Manager UI.

The Issue:
Even though I am explicitly sending Opportunity and Customer data in both the createTransaction and updateTransaction XML payloads (e.g., opportunityID_t, opportunityNumber_t, customerName, etc.), these fields are completely blank when I query the transaction or view it in the UI.