Hello, I'm no expert by any means, but I thought I would say something since this has gone unanswered for some time. I don't know what you are using, but I am right now using Shopify's "Email" app for my email automations. This app says that it only keeps workflow runs for 7 days and then deletes them. It may be app specific, but I'd look around inside the app and see if there's any "fine print".
