Why would the thrust lever of the left engine reduce?
The preliminary report says that the "thrust lever of the left engine started reducing, while the thrust lever position of the right engine remained." Assuming the RH remained at a high power setting, I cannot see whey the LH thrust lever would start coming back.
The A/T drives both levers together, if one jams, the other lever still moves. If the motor fails, the levers will stay in the last position. Both levers run off the same air data source so one would not be commanded to drive back.
I can see no reason why the A/T would drive one lever back, or if the clutch slips the lever would fall back itself.