This FGC property has no effect as a trip level (which is fixed inside the state machine card).

You set this to be the voltage level that trips the converter. The trip level is 0.6V (=0.006A) for SIRIUS and is defined by the rail of the comparator in the state machine card.

When you set this property correctly then fgcrun+ and powerspy will correctly display the Iearth command: G MEAS.I.EARTH.