I own 3 P3ATs, 2 fg and 1 sg and they all have problems with extended magazines. Two are back at KT right now. I believe that I caused this problem by my old habit of finishing the mag insert with a good palm smack. I try not to do it anymore. This never caused a problem in my other guns, but this one is a polymer mag catch with a steel magazine whose sharp edges can tear plastic.
The extensions (whether Bersa or KT +1) force the bottom of the magazine back and holds it there. When gun is fired, recoil pushes top of magazine back so that whole mag is set back about 1/16 inch. Weakened mag catch can't hold the mag anymore and it falls out. After a while it won't hold non-extended mag either and it start falling out every 3 or 4 shots. You or KT must replace mag catch.
I have devised a method for keeping the extended mags reliable and it has worked well for me. First replace mag catch. Go to a store like Home Depot and buy, for about $1.50, a clear plastic wall-corner protecter. It comes with self-adhesive strips. Cut off a 1/4 by 1 inch strip with scissors. Remove protective paper from adhesive and affix to bottom rear of magazine. This prevents mag from moving back at rear. If plastic strip is too thick to let mag go in easily, you can sand it thinner to your preference. Packer.