First, the monitor should cover the whole simulation spans in order to not miss any diffraction.
For such 1D grating, it is better to use 2D simulation, as the other dimension is infinite by use of periodic boundaries in 3D simulation. In addition, for such 3D simulation wih 1D grating, the grating analysis can sometimes give fake diffraction orders in the uniform plane. However the diffraction strength is zero.
To get the zero-th order diffraction, please refer to this example: Diffraction grating