Randomly changing midi value for a fader

Hello everyone !

I have tried for some time to set up my faderport 1 motorized fader : every button works well, except the main fader. When I move the fader, the midi learn shows the correct velocity, but the midi input value keeps taking random numbers, which makes it impossible to make it work in ableton.

Do you have an idea to fix this randomly changing midi input value ? It also appears on another controller for a endless knob.