ESD strike on exposed circuit ground pin
For battery operated device, you will eventually find it to Not Discharge as it's not grounded. Which is consider a passing scenario for ESD. Unless the technician help you to discharge to ground after every zap.

To allow the ESD to neutralize faster, add a 10uF cap between Vbus and Gnd. Nearest to the USB connector. When zap +8kV to Gnd, the cap will discharge a little to take in some ESD energy. What I would not want is to find the discharge travel from my ground, goes to a ground pin in some component and then exit the component's supply pin.

Ceramic base TVS are coming and their reaction time is much faster than conventional diode base TVS.