I'm updating this thread to report its now been almost 90 days since I've noticed this problem. My issue appears to be fixed!

I can only guess that I needed to give it more time for a higher concentration of the new fluid make its way to the torque converter.

This past weekend I tried repeatedly to duplicate the problem by traveling on a hilly road and carefully setting my speed so as to put the transmission into that area where it wants to downshift at light throttle.

All good now.

Hopefully this thread helps others with this issue, I'm at 76,000 miles currently and it seems 100,000 miles on this 'WS' fluid would be waaaay to long.