My PolyTune 2 Mini does not appear to change brightness at all. Is this a fault with the pedal?
No. unlike the PolyTune 2 standard size, the PolyTune 2 Mini (or Mini Noir) does not have an ambient light sensor. It was not implemented because the display is bright enough to cope with all different kinds of ambient light situations.
Therefore, we apologize but there is indeed a mistake on the user manual regarding this particular point.