I understand the unit fell off and was lost. What exactly failed, rivet, buckle, prong? Was the unit secured by the prongs passing through the collar? Were the prongs changed or otherwise checked for tightness?
The reason I bring this up is I recently installed an E fence and the inner prongs pass through the collar and are secured by the outer prongs, both were loose out of the box. I imagine it wouldn't take long for them to completely loosen and the receiver to fall off the collar. I wouldn't count that as a defect.