Reservation disappears!!

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Hello Experts,

Many occasions I have observed that "Manage reservation and Picks" do not show reservation even though Sales Order fulfillment line shows reserved qty and sales order line is awaiting shipping. This come to attention when we are running Pick Release process with Prior reservation as Yes and then try to figure it out why it is not getting pick released, then we find out that there is no reservation

I am not able to figure out when does the reservation disappear? Does any of sales order revision process remove reservation? It does not happen all the time.