Airspeed constant offset during flight

my Matek ASPD-4525 differential airspeed sensor is reading constant 5-8m/s too low speeds, even after calibration. This behaviour is the same from 25m/s up to 75m/s ground speed.
The pitot tube is mounted on the nose. This happened in two flights with different planes who use the same kind of sensor. There was almost zero wind and I flew circles, so wind is not the issue here.
As far as I understand, the Airspeed should recalibrate itself during flight (adjust the scale), but instead the airspeed failsafe flag appears and the vehicle falls back to using ground speed.

How can I make my Airspeed usable? Do I need a different kind of sensor?