Guilin Sun
Ansys Employee

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.