You are right that the extra mesh is required.
since it is discrete, we cannot use the continuous view to explain it. Internally the 6 field components are not collocated. however by default the exported data from the monitor is interpolated to one location so you will not notice the difference. Lumerical gives customers the option to export the raw data. However, without proper treatment of the raw data, the power calculation will be incorrect, even though the difference can be very small. Please refer the analysis group ("power absorption advanced").
Due to this reason, the periodic BC should lay on uniform objects if possible.
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