This is common when you change the source spectrum but do not modify other settings. It is usually due to two causes:
1: when the min wavelength changes, the mesh will be changed in general. This is because the min mesh size is determined by the min wavelength, or more specifically, the min wavelengh in highest refractive index material.
2: when the source spectrum changes, the material fitting will usually change, except the materials are non-dispersive.
To avoid this, you can either tick "set simulation bandwidth":
or set mesh generation bandwidth and material fitting bandwidth: