It did not last long enough, only few 10th of second, as it was due to very short flight spikes exceeding alpha treshold.
IMHO, if that is the modus operandi then it needs looking at too..
Any 'spike' triggering the Stall Warning, should ensure that it sounds for a minimum time. (say 1 or 2 seconds) OR continue until AoA is below a preset delta below trigger point (say, 1/2 degree above 20,000 ft and 1 degree below)