Completely blocking off the output messes up the resonance of the
housing, this can cause excessive vibration of the piezo element and it
can crack. Obviously scotch tape still vibrates and passes the sound
out, it is not a complete block of the sound outlet.

We have even received a service bulletin from one particular medical
device manufacture due to the high failure of the peizo buzzers and the
cause being the nurses putting the thick tape over the buzzer hold
because it was too loud for them, completely blocking off the hole.
Not to mention the fact that the nurse was in effect tampering with a
patient safety alarm in the first place, what they did would eventually
completely disable the alarm putting the patient at even greater risk.