For what it's worth....
On my first cb guitar I mounted 4 piezos under the soundboard still in their plastic casings. These were fitted with both a layer of Lidl repair glue and 2 screws in each using the casing holes (2 per piezo). I get massive feedback with this guitar similar to that which others are reporting.
Somewhere on this site there is a video about using a capacitor in parallel to reduce, but not eliminate, feedback.
My first guitar is well sealed-up so I can't easily get into it to try a capacitor but I certainly will deploy one in my next project...and on this occassion I will just glue the piezos (out of their cases) under the soundboard...This time I plan to use 2.