By default, the solver uses the first mode it finds. When the material is changed, the modes may change. For example, in one waveguide the first mode is TE, but in another material, the first mode can be TM. In addition, when the modes are degenerated it can be either of them. If you want a specific mode, please choose "fundamental TE" or "fundamental TM", or you use "use select" and calculate the modes, then you can choose whatever the mode you want.
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