Sorry to reply you late due to ceratin circumstances.
From your results, it seems the S parameter has some issues, at it changes violently. It might have higher differaction orders. Please use farfield option to pick up only yhr zero-th order.
In addition, dupicating other's result can be very challenge, as the original authors may not give clear details. Sometimes people use the same concept but different math definitions. For example, left-and right- circular polarization can be opposite. TE and TM modes have the same issues.
Please refer this post to check if everything is the same as the paper:
Ansys Insight: Why my simulation result is different from published paper or experiment?
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