This is looking like second stage power turbine failure to me, it’s easy to miss identify an engine failure in this condition and not something we train for since simulators generally don’t accommodate this condition.
How we monitor the condition of this turbine is when takeoff torque is within 2% of target torque for takeoff check, assuming this was within allowable limits at takeoff then as the second stage turbine degrades the torque will fall away until it drops to the auto feather trigger value, usually around 50%.
This is where the confusion starts, the gas generator is still functioning normally the feathered propeller is still being driven by the seriously degraded engine, you would expect to see a prop rpm in the order of 30 to 40 percent and a torque value quite possibly exceeding 100%, with no engine failure indications apparent, if all your looking at is the torque gauges, left at say around 98% right engine more than that and an airplane performing as it should on one engine, one can begin to see if we are honest how easily it is to misidentify an underperforming engine, particularly if you.ve never seen this condition before.