A spring or the plastic spacer/insulator could have broken. As I recall someone had a broken insulator piece not too long ago.
Too many years since I had mine apart so not remembering exactly where the detent is in the housing. Over to the right of the switch in this photo is suspect. Does the spring and round retaining clip on the switch stem help keep a bump on the end of the cancel assy pulled down into some notches within the area of the housing under the red oval that could act as detents?