I tested a dispersive material but could not duplicate this issue when I used:
setnamed("RCWA","use wavelength spacing",1);
and override the material fitting wavelength range to be the same, for exmple 0.4~0.7um, whereas I tested 0.4~0.5um for 6 points,and 0.4~0.6um for 11 points for a silicon slab with air holes (adpoted from online example https://optics.ansys.com/hc/en-us/articles/4414567929235-rcwa-Script-command the script "rcwa" is depreciated now).
Maybe you can choose visualze the max frequency point and see if they agree.
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