Position of center of light gets moved in PSF postprocessing

During the debates about centering and positions of the 2d PSF I have repeatedly claim that my PSF is centered so that the chief ray is the center of the created image. This is true for the pure PSF which I create, but afterwards I apply ``postprocessing'' where I convolve the image with the fiber size, grating effects, scattering and charge diffusion. All of these effects are symmetric and should not modify the center  of light. Observing the result we see that this is not the case. This is probably because I was not careful about this, as centering was never issue for me until now.