The Aqualloy or Aquamet or Nitronic shaft is a harder material than the 18-8 set screws so if you tighten the set screw more than once, as in making a slight re-adjustment, there is no bite left on the head of the set screw to score the shaft and lock it into place..

I recognized this issue perhaps 14 - 15 years ago and sourced myself a Ruland "clamp collar", as added insurance. PYI liked the idea so much they now sell their own clamp collar...

Used and new set screws.

The set screws are a critical component of the PSS system. You should always use two set screws in each hole. The top one is used to lock the bottom one into place and prevent it from accidentally backing out.

If you tighten the set screw to torque once, that is your one use. For example if you are installing the PSS and adjust it, tighten the set screw then realize the adjustment was wrong you now really should use a new set screw. If you look closely you can see how the used screws flatten. They will not score the shaft, like when new and sharp, to lock the rotor in place with a second use.

If you are in an absolute emergency you can swap the screw that had been the top "locking" screw for the bottom one, but this is still not advised as a regular practice. PYI generally recommends using new set screws with each tightening and loosening of the set screws.