Old Farang, I don't claim to be a scientist or an engineer, I am just passing on what was taught to me many years ago by people who were; if it is wrong then so be it.
I know how a freewheel works and I also know they don't always operate perfectly - some friction drag is quite possible if the sprags don't completely disengage.
Perhaps you can come up with an alternative answer as to why some have experienced higher RoD in auto without the engine running compared to with it at idle.