Hi
most of the helicopters may have rotor stall during vertical descent with more than 500-700 ft/min and fwd speed less than 30 kts but this only matter when your blades are with positive angle of attack , so if you are in the same conditions but has your collective fully or almost down (0 or very little pitch ) this shouldn't happen ?
My consideration are in regards to approach to land technique with very dynamic 180 degree turns in forward flight like for ex. on below video (40 sec.) - this rotor didn't stall because the airflow was from below (desending) and it has some fwd speed .