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Advanced Web Machinery

AppSync subscriptions may miss events even after receiving a start_ack confirmation. The start_ack message doesn't guarantee that all future events will be delivered, creating a temporal dead zone where updates can be lost. This breaks the common pattern of subscribing before querying current state to ensure no missed updates. The issue is particularly noticeable in tests and async workflows where subscriptions are created just before events occur. A bug report has been filed with AWS.

AppSync subscriptions: waiting for start_ack can still result in missing events