Guilin Sun
Ansys Employee
You are right that the phase at a specific wavelength should be fixed in physics.
However, the resulting phase is highly dependent on the mesh size due to numerical dispersion. When the source start wavelength changes, the mesh size will automatically change by default. The mesh change might also be due to material fitting change: when the spectrum changes the fitting may lead to different material data.
To alleviate this problem, please fix the mesh generation and material fitting wavelength range. You can use this:

or more specifically, set them in mesh generation:

and fitting:
They should lead to the same result. Please try.