The preserving bag doesn't seem to take effect on newly added item stacks. The timer on the item is updated, however. For example, adding a stack of cooked meat, the meat says it has an expiration of 1 hour. Exiting and immediately re-opening the bag and looking at the same stack again, the time will have reduced by a large amount of time. It seems after the first item of the stack spoils, it starts preserving the items correctly.