Where is this additional “agricultural traffic free” sign supposed to be? I've just gone through my mapillary photos from the summer and can't find one either in the north at the Schmitterbrücke bridge nor in the south at the customs in Hohenems.
Or check this one in the middle: https://www.mapillary.com/app/?pKey=810729587830619
https://www.openstreetmap.org/way/1169123130

Do you have photos of the signage you think you know there?