The thumbwheel is physically connected to a potentiometer by a plastic pin which passes through its center. This pin may be broken. In particular, if someone were to remove the face of the controller without first removing the pin it is very definitely broken, since the thumbwheel is part of the face, and the potentiometer is on a board in the housing.

Is your pin broken? You may get a sense for this by judging how easily your thumbwheel turns, as a broken pin would obviously allow the wheel to spin with less resistance. Further, you can remove the climate controller and pry the pin out and examine it. There is an opening on the left side of the controller to access the pin.