Guilin Sun
Ansys Employee

It is most likely due to material fitting. Although RCWA is frequency-domain solver, currently it still uses multi-coefficient model:

so when the wavelength range changes, the fitted result may be different. For the same geometry, different material will lead to different result. So please make sure the fitting wavelength range is the same for the two cases:

Please try and check if the result is about the same, if not exactly the same.