Usually FDTD is better used for broadband, even though you only need results at a single wavelegth. If you only set one single frequency you may not know if it has resonance and since the actual injected signal is broadband, meeting the autoshutoff min does not mean this frequency reaches the same level.
To allievate the issue you faced, please tick this option:
and specify the min and max wavelengths for simulation. In this way, the simulation settings make sure the material fitting model is the same whether it is at the single frequency or broadband, and the mesh will keep the same at the same time. You can also separately set them: the material fitting range (advanced in material explorer) and the mesh generation "override simulation bandwifth for mesh generation" shown above.
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