Urgent question on ABS

Hello, I still have an appointment with Tüv today, only I still haven’t managed to solve my braking problem! Brake discs/blocks in the rear and front are NEW! Brake fluid has been changed in the workshop! (I thought that there was air in the system, it would be it! Unfortunately, there is no problem) Problem is still! I drive slowly 10-ca.50kmh and then the ABS system switches on in the front and I get a kind of vibration in the pedal! If I drive faster and brake, everything is normal! e ABS light is always off! Even if only the ignition is on! Is that correct??? If no, where should I start looking for the error?? (The lamp itself is healing! Have measured the speedometer for another reason!) If it has to be so, then I have to keep looking! I’m just cleaning the sensors at the front! A quick but helpful tip would be very convenient! 😉 Thanks in the trust MfG The Polo2 86c