The change in capability in auto follows the power-on flight effects. The power needed to hover is higher because of density effects on the rotor, so the "power" needed to stop the auto is higher, thus the rpm droop with collective pull is faster, and the collective pull is less effective.
The steady state rate of descent is the same (a trick of aerodynamics) but the collective pitch setting to get that ROD, and set the rpm properly, is much higher.
Altitude autos at gross weight are some of the most difficult things test pilots do.