the "Medium" difficulty has been replaced with the "Pro" difficulty months ago, on last September exactly.
At this occasion, I revised a bit the CPU comportment and placing at volley.
I just tried the play against weak volleyers, in "Pro" and "Junior" difficulties, and they didn't miss any smash on the lobs I did on return, so I guess my changes of last September has removed this exploit ...
So you should update your Tennis Elbow with the latest version available on the website.
Anyway, thanks to report this, and if you find any other similar troubles, don't hesitate to let me know !