I also have owned a 20b lachenal for just a couple of months, and more than once and on different buttons, a note sounds all the time with no button pressed. It looks like the button is lifted slightly. I’ve found this is caused by the U shaped pin that holds the rod down has lifted a little too far out of its holes, then the spring pushes the rod and pad up, leaving a gap under the pad. Just pushing the U shaped pin back into its holes has fixed it for me, and settles the button back to a better height. It happened again yesterday, i plan to dab some glue in the U-pins holes to try to keep it down.
Sorry, I don’t have the terminology down yet. Is glue (superglue?) the right thing to keep these pins from popping out?