What’s kind of expression pedal are you using? Is the potentiometer rated for 10 kohm – 25 kohm, the H90 sweet spot?

The first thing I would try is calibrating your expression pedal in the system menu > I/O > Calibration. Make sure you’re getting a zero to 100 sweep range and it says zero when the pedal is in the heal position. Auto engage is dependent on the expression pedal position so ensuring proper operation is crucial. The only other parameter that may be affecting performance is the Auto Engage Hold Time: If Auto Engage is on, this parameter allows the pitch shifted effect to remain on for a period of time from 0 to 1 second. This can help the auto engage parameter from disengaging abruptly. It would be worth exploring if different settings work better for your setup.