Unfortunately, piezo pickups will eventually fail, not a matter of if, but when. A piezo pickup works by being ‘deformed’ (alternately squashed and stretched microscopically) by string vibrations passed through the saddle. Made of ceramic, these vibrations create tiny fractures that over time will diminish the signal they are able to produce.
(great article here if you want to learn more. https://hazeguitars.com/blog/acoustic-power-the-magic-of-piezoelectricity)
I have three Variax guitars, the Variax Acoustic, the JTV89-F, and a JTV-59, I love these guitars and have gigged with them for years, but all three have this piezo issue to one degree or another. Originally I used the workbench software to adjust, but found that tedious. I now have a Helix floor which makes this issue much more manageable.
However, adjustment will only work for so long, eventually they need to be replaced. The good news is the process is relatively simple with a little patience and time. I have had to replace two of the piezos on my JTV-89F with very good results. You can buy the parts here from Full Compass (https://www.fullcompass.com/brand/ln6-line-6/accessories/parts-replacement-service/musical-instrument-parts/guitar-parts/ Pay attention to which string you are replacing as there are three specific part numbers. On this guitar you remove the whole saddle, the wires feed through the tremolo cavity to a wiring assembly in the main control section, no soldering required.
Here is a video I did playing some acoustic patches after replacing two piezos, I think it sounds great. Although I bought a couple spares for next time... Good luck!
P.S. If an alternative pickup design could be developed, the Variax would truly be the most amazing guitar ever! (just saying) :-)