zone transfer request target_project_id update inconsistent

The API reference guide shows "target_project_id" as an optional setting when updating a zone transfer request.

If the zone transfer request does not have a target project ID assigned, you can set this field to a new value.

However, once it's set, you cannot change it again.

{"code": 400, "type": "invalid_object", "message": "Provided object does not match schema. Keys ['target_project_id'] are not valid for transfer_request", "request_id": "req-c250efd4-869f-446d-ba9d-77aeb73ddc4e"}