By adjustable lift arm, do you mean the center top link, or do you mean the side leveling adjustment on the lower (typically right side) link? Reason I ask is as far as I know the two lower links are the lift arms, with the adjustable one being a small turnbuckle; and the top link has the stop nut, it is also sometimes known as the draft link (engages the tractors draft sensor for draft operation).
If the side leveling turnbuckle has a stop nut on it also, I wasn't aware and learned something.
Generally a stop nut turning with whatever it is supposed to stop is just a case of not having them tightened against each other enough.
My brother in law's tractor has several turnbuckles on the back and has also had issues with vibrations spinning them out of adjustment. His fix: rubber straps. He has straps hooked to the two sway stabilizer turnbuckles and on his leveling adjustment turnbuckle to keep any from turning.