In this circuit the bidirectional transzorb must be exchanged by an unidirectional!
The 10pF cap and 1M resistor should high voltage versions.
Add a small capacitance across the basis emitter junction of NPN transistor.
What cable do you use for connecting the piezo to circuit? Maybe it's better to postition the 1M resistor, the 10pF cap and the unidirectional transzorb next to the piezo, not to the circuit. You can also add a RC-lowpass filter at output of this arrangement. Then, only rather low frequent signals are running to the micro board, with the NPN transistor and its associated resistors at input. In this case do also add a small capacitance across basis emitter junction.

Keep the piezo and all involved fast signals away from the microcontroller. The trasnzorb must not sit next to the micro! There's always enough stray capacitance to inject unsane spikes into the micro's port line. Only proper shielding and filtering would solve the problem. But with the transzorb outside and the micro inside of this shield!

Kai