It is better to look at the deleted messages on the queue. The messages in flight is just a snapshot at a certain time. If the message was deleted already because the task did not take long to complete, then there would be no message in flight. I assume your task did not take long to finish ( 1 second maybe). And what are the odds that the messages in flight metric is updated at exactly that time?

So in order to be sure just check the deleted messages on the queue or as a way of testing it, add a sleep of approximately 60 seconds to every task. This will make the task take longer and allows you to see that it is in flight.