Bell ringer - the rotor stall would only occur following a real loss of power, not for a practice autorotation.
FW pilots tend to react to a stall by pushing both hands forward (stick forward and full throttle) which translates to lever down and forward cyclic in a helo - if you do that for a practice autorotation and are even remotely ham-fisted, you can quickly get to a low G, mast bumping scenario.
You can get tail strikes from both low Nr (rotor stall) and low G with associated mast bumping and undemanded roll. FW responses to emergency (or practice emergency) situations can be fatal in both scenarios.