Are you doing 2D simulation? otherwisde you should use a 2D monitor not a 1D line monitor.
Which field component, Ex or Ey (transversal component) is the dominant?
Have you considered my suggestion to use S parameter? Since this is waveguide and the mode does not have a flat wavefront, you cannot use the phase at one point or a line or a plane to quantify the phase.
Only S parameter can have one unique value of phase, after propagation phase compensation.
I hope this helps.
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