In the Simulation Parameters dialog box, in the Global Data tab, there is a field called “Number of displays”. The default is 100. It is the number of times OptiBPM samples the problem in propagation to determine what to draw. You are displaying a grating which has lots of ups and downs. With only 100 samples, some of the ups and downs are missed. Increase the Number of displays to 500. Your grating will appear as it really is now.