FDE uses a random seeds to solve the eigen values. So even if you input an exact value of the effective index for a certain "higher order" mode, the solve will search the solution around it, usually larger than it to begin with. So when you set PML BCs and give a search range, you will still need to set a large number of trial modes, say 999. The solver finds a lot of PML modes and then throw them away and keep the propagating modes only. Those PML modes are counted so you will need large number of trial modes.
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