Hi Rusty_Nails,
I have had the 1 lap race option working from the first trainer released but have never seen the actual amount of laps change until you are in the pits right before the race! It comes up as 1 lap on the little screen that is mounted on the front of your car.
I also activate the option before entering the race menu while in the trailer then moving the slider, deactivating the option then doing it again in as you where just to be safe!
To be on the safe side, it may be worth activating the option before the race, moving the slider, and keeping the option ON until you are sat in the pits. Just in case anything reverts the laps back to normal during the loading sequences (it shouldn't, but it's possible).