Are you sure the nut actually backed off? Did you measure the lever height?
I ask because there is supposed to be play on the lever. The nut should not be tight down against the lever feet.
The true measurement to check the nut is just based on the lever height. When you look at the edge of the can surface, you should see the lever sticking up and see the plastic beads completely, but the lever should not stick up any further. I hope that makes sense.
You can also use the same lever gauge used on a Mistral to give you a reference.
I agree with DerekRalston, I have never seen a nylon locking nut move.
Back in the 90’s I even read a report on a study (I think it was from the Air Force) were they tested the torque value of nylon locking nuts after they have been removed and were being reused. They determined that it was safe to re-use the nuts several times with adequate locking torque. This was even for use in an airborne vibration environment. I can’t remember the maximum number of time they decided was acceptable to use them, but it was more that most of us will consider reusing them. I don't know if they change their service procedures either.
I buy the nylon locking nut in bulk, but still sometimes re-use them in my own regulators if they feel hard to turn. My experience using a torque wrench on nylon locking nuts is that they keep a substantial amount of torque even after they have been reused a few times. But they are cheap so if you feel like replacing them, it is good. YMMV
To be honest, once I adjust the nut in an HPR, there is really very little reason to readjusted. I have just changed the seat when it was too deeply engraved and just put it back together and took it diving. I very rarely with take apart the nut on the second stage, there is no need in most cases.
Buceador con escafandra autónoma clásica.