If the client clock is slightly ahead of the brokers clock a message could be expired on the client but not considered expired on the broker.
When the expiry ACK is sent to the broker it checks if the message is also considered expired on the broker side. If the broker clock is behind the client side clock the message could be considered not expired on the broker and not
removed from the broker's dispatched list. This leaves the broker reporting a message inflight from the broker's perspective even though the message has been expired on the consumer(client) side
The broker should treat the expired ACK as the authority on whether a message is expired and process it as such regardless of the broker side clock.