Actually it works as expected when there is something already drawn inside the PGraphics, but it send me an error message when I apply a 'background(0,0)' directly after creating the PGraphics.
It's not really a big problem , but it's a good thing to know
(My poor solution is to add a beginDraw-beginShape-endShape-endDraw directly after the creation of the PGraphics instance, just to be certain that there is something to erase ; and it works perfectly)