IME, when on an award ticket in IRROPS, and the fare class changes to a revenue-class (i.e., N to Y, or R to F), RDMs and MQMs accrue. However, I can't recall ever retaining the NO FF CREDIT designation on the new BP. Is it possible for a GA to rebook you in a rev class but somehow block mileage accrual? If you've got some time on your hands, see if you can find a kiosk to reprint your BP and see if the notation disappears.