My Guess...
We are used to thinking in terms of an analogy to true airspeed increase with DA increase, but the flaw in that thinking is that in the case of TAS, we are dealing with the vehicle being driven by thrust through the working medium of the air.
In the case of autorotation and rotor RPM, however, we are dealing with the rotor being driven by the working fluid, i.e., the air.
When that working fluid becomes less dense, i.e., the DA increases, there is less energy imparted to the rotor's autorotational driving region in the autorotational descent. Therefore, for a given autorotational condition, i.e. airspeed, the autorotational rotor RPM will be less for high-DA air than it will be for low-DA (that is, denser) air.
What do you think?