You are correct that the lap position should hold the train-line pressure at what it was set to. I'm assuming that you are not using self lapping brakes (found on many modern locomotives) and that you are making a reduction of at least 10 pounds (69 Kpa). Have you checked to see if this is happening on more than one locomotive and map? My guess is that the specific locomotive you are using has an error (or a bad value) in the brake system somewhere.