Inventory shows in stock intermittently

Support escalates at 13:00: customers report that products they marked out-of-stock are still showing as 'in stock' on the storefront, leading to oversold orders, but only on some page loads and it 'sometimes corrects itself after a while.' The storefront reads inventory through a Redis cache with a 5-minute TTL. An hour earlier, you deployed a change that moved inventory updates to an async background worker queue to speed up the admin panel. There were no errors anywhere. How do you triage and fix?