Soft starter and inverter in creating shaft current

The way a soft starter works is NOT AT ALL like an inverter. All it is doing is retarding the gate (turn-on) time of the SCR so that only a portion of the AC sine wave gets through. Inverters create pseudo sine waves by PWM firing of DC with high speed transistors. Totally different and there are no "pulses" with Soft Starting, so there are no additional shaft currents created. The soft starter is NOT the cause of your fluting.

Fluting is more often a symptom of vibration. Shaft voltage discharges cause pitting, pitting causes vibration, then vibration causes fluting. But just because there is fluting does not mean there is shaft voltage discharge. Fluting existed long before frequency inverters.

In any inverter, the output side neutral is derived one. Which means this can not be exactly at zero potential since you need a non zero error signal to bring the neutral point close to zero. This non zero neutral will induce voltage across bearing film. Probably you need a voltage of mili volt range to create sufficient current over time to create fluting. That is my understanding.

Insulating only NDE bearing of motor may create alternate path through driven equipment bearing. Isolated coupling with driven equipment as well shaft grounding brushes may be options along with insulating both bearings.

Most of the soft starters in the power side, have SCR´s than by the frequency than they switch did not create a parasite currents (inducted) in the shaft, just in the frequency inverter then has a PWM technology by the switching frequency, and the level of the DC in the inverter creates this kind of problems, must be a static electricity created by the load than is moving and the only path to discharge is the shaft in the motor thru the bearings.