I have added an Auto Generated Number field in the Deal layout.

This is in a section which is displayed conditionally, only when the Stage is "Deal Won".

Problem I am facing is that whenever I am creating a fresh deal or a new deal from Contacts, the order id is getting generated successfully, but whenever I am doing it on a deal already created, and trying to edit it and changing the stage and then saving order ID field is blank and shows n/a.

Any idea why wouldn't it generate for existing deals?