I think it was Cannonball who mentioned a clinic or presentation he saw by Gerry Glasco a while back. Glasco talked about the value of tee hitting without a ball - just hit the top of the tee. As you discussed with your hitter, using the flight of the ball off a tee as your evaluation for swing quality can give you a false positive. Removing the ball flight from the equation puts the focus on evaluating just the hitter's mechanics.
My point was that a positive result off of a tee does not equate to a positive result off of live pitching.
A negative result off of a tee is generally indicative of an issue.