I have also noticed that the mode convergence sweep doesn't converge (when using conserve energy): 

Interestingly, here (“CVCS” or “none” for the taper waveguide coupler (ansys.com)) it is recommended to make energy conservation passive, whereas here (EME solver - Simulation object – Ansys Optics) it is mentioned that energy conservation should be enabled when simulating a periodic structure. So which option should I use? 

Best regards,