Error saving "renew until" during renewal flow – ValidUntilProlongation is not writable

Description:

We have customized the Angular frontend in our Identity Manager portal to support different date handling for new and renewal requests.

Expected behavior:

• For new requests: Approver can modify ValidUntil.

• For renewals: Approver can modify RenewUntil (mapped to ValidUntilProlongation).

Problem:
When an approver tries to save a modified renew until date during the renewal process, the following error is returned by the backend:

This seems to indicate that ValidUntilProlongation is considered read-only by the API layer.

Question:

• Is there a way to make ValidUntilProlongation writable in this context?

• Or is there another recommended way to handle date modification during renewals?