Why, because on the display P/Y/R when pitch is around 83 degrees, as for grabbing HST, that A angle appears on the value of Y, not on R!

Well, at the Euler singularity there's no way to define a difference between yaw and roll. So are we talking an Euler singularity issue here?

I was trying to describe what I was seeing in that the jump corresponds to the shoulder yaw angle and that must be translated to the end eff roll angle in some manner.

The end effector roll angle is what is shown on the display. That's the only property of that type as far as I understand my code. Now, if the problem is at pitch > 83 deg, then computing what is roll and yaw is blurry - the values you see depend increasingly on artifacts of the numerics, and the animation framework that determines where you see the arm (nested matrix equations) might do it differently from the JSBSim numerics (trigonometry) that determines where you see the payload (via translation animations).

So is that what you're seeing - different numerical artifacts at the Euler singularity?