I'm not really sure you can rely on the resolution of that speed indicator. In any real situation the speed must have increased at the knee of the descent, unless the engines were perfectly throttled back to compensate. That straight line is too perfect.

Unlike the altitude which is reported in real time by the Mode-C of the transponder, velocity may be calculated from position and run through a low pass filter. There could be a significant delay from when the true airspeed changed to when it is displayed on the graph. We are talking a total time increment of only 5 minutes, and that constant output with a sudden drop at the end really looks like it is an artifact of the calculation.