Synthetic airspeed shows strange data

I tried to use the synthetic airspeed estimation with my glider and found a strange data.
I flew the glider with the Autotune flight mode and applied several elevator up and down controls that might be sufficient pitch angle change for the synthetic estimation.
Below is the flight log data of this flight. Altitude, GPS speed and the synthetic airspeed(CTUN) are plotted. A little bit after 13:24, both GPS speed and the synthetic airspeed oscillate violently. These are because of the elevator control to perform the autotuning. The strange point is that the synthetic airspeed gradualy drops down to almost zero. That is well below the stall speed of the glider. Could someone teach me why?