....Happens on any note I play on teh G string, but is most noticeable in upper octave....
maybe the piezo is catching a little also the slightly vibrating part of the string on the left of the fretted note..
try dampening that part of the string with a free finger or two, and see if that helps..
the G is the guitar's most resonant plain string, and normally accentuates more than others every little nuance
when you play at the 12th fret you have 2 almost identical halves of the string beside the fretted note, one on the left and one on the right.. we expect that the left side doesn't vibrate, but often it does a little, and can be catched by pickups..
since at the 12th fret there are 2 almost identical string halves you get something similar to when two strings ringing together are not perfectly in tune
if you do some tapping notes especially on the 3 first plain strings from around the 12 fret area onwards, you can hear both sides (left and right) of the strings ringing together..
let say that the more free string length there is on the left of the fretted note, the more easily it can passively vibrate