PGraphics unpredictable and unconsistent behaviour in Android mode

I'm developing an Android live wallpaper and I'm struggling with various problems the PGraphics objects have when rendering. The main problem is the impossibility to clear a PGraphics object with alpha values every frame, and everything I've tried works lika a charm in Java mode.

I'm using P2D as main renderer and the default for the Graphics object one because a whant to obtain a scaled pixel effect using the OpenGL nearest neighbor trick, but that same trick cannot be applied to a PGraphics rendering (so I use default + noSmooth() for this one). After Drawn everything on the PGraphic object, I scale the image on screen using the copy() method.

The result should be an image composed by PImage and PGraphics objects, but these last does not clean after a clear() or background(0,0) call so the image upon them is additive. The clear() call in particular gives me a tiny performances drop, while background(255,255) does its job so it's working fina as method.

I've tried every possible turnaround but nothing has the same performances/appearence as should be in this common way not working because of these problems. I don't know if I'm missing something or if I'm too stupid to figure out a simple solution.

here is a piece of the code that works on Java mode and does not work in Android mode:

pioggia.beginDraw();
pioggia.background(0,0);     //or clear()
for(int i = 0; i<gocceNum; i++){
   gocce[i][2] = (gocce[i][2]+gocceSpX*0.25)%back1.width;
   gocce[i][3] = (gocce[i][3]+gocceSpY*0.25)%back1.height;
   pioggia.set((int)gocce[i][2],(int)gocce[i][3], #355B4A);
}
pioggia.endDraw();
copy(pioggia, (int)quantize(disp*0.4, scalaH), 0, sw, sh, 0, 0, width, height);

"pioggia" is the PGraphics object, gocce is a 2D array of float values. Any help or hint would be super awesome ç_ç