Citrus
That's bad luck,
I've been using one of those carriers for over a year. bought it on Ebay. It has a Gnog trunk bag bolted to it containing a Ping 24V 15 ah + a full set of tools and spare chain for rural off-road riding. The total weight of the trunk bag is 5.2 Kgs.
The route is the rural section of route 45 Swindon to Marlborough, 22 mile round trip. In places it is so rough I have to stand on the pedals with knees bent jockey fashion to stop my teeth shaking out!. I've ridden that route 5 times in the last 14 months, and the carrier is showing no signs of fatigue, You really have been very unlucky with yours.
Or should that be...I've been lucky with mine!.
All the best
Bob