tis really common for the ARB mount captive nuts to break there weld in the box section. the easy repair is to drop the bolt from the inside (can drop it in from the access holes) and put a nut on the outside. the alternative is to lift the carpet in the footwell and there's lil grill that vents above the captive nut. remove a couple of the slats and you can get a socket on from the top

job done, I've done it both ways numerous times and never a problem

Rob