I think we should consider the difference between FIG and autorotation.
Firstly, the situation you refer to when you put the collective down and conduct a PFL is not an autorotation unless the engine (gas turbine type) is not running. You are in fact conducting a Flight Idle Glide (FIG) during which there may well be a significant contribution from the engine towards the rotation of the free turbine. During a genuine autorotation, the autorotaive force that turn the head is a result of aerodynamic forces only which results in a higher rate of descent during an autorotation thatn during a FIG. I have noted this on a number of aircraft ( when conducting a formal flight test)by selecting gound idle or shut down during steady FIGs and noting an increased rate of descent ( about 200 ft/min on some types). I do not suggest you try this unless your current regulations permit it as I do note promote DIY flight test.
Obviously it is only a problem where a large contribution in FIG means that PFLs are not representative of the real engine failure case. Any thoughts?