Yep, and you'll see many, many fenders bent outward, just past the rear frame, where all this loading occurred! Mine's included, from my early days of setting my oversized backpack right on the tailight and my rider using it as her backrest.
I did one thing that helped reduce that breakdown from the cantilever designs (on several bikes, not just mine) because they were usually strong enough, so long as they did not twist: I made a wide 1" x 1/8" thick metal (aluminum) strap, with the ends turned up at 90 degree angles just far enough to reach the back of the rack, with the middle of this 'bracket' bolted underneath the 4-bolt taillight's mounts. My current setup of Krauser rack/bags/tourpak is like this, has been for over 30 years. (Will try to return a pix, here...) What it does: it stops the side-to-side twisting of those tubes, which caused the fatigue, which left many a rack on the road. And it puts very little load on the fender itself: I made it with slotted adjustments on the ends so I could pull the fender UP slightly when tightening the bolts.
Like you, MC: I broke 2 racks in 2 years when they were not bolted down back there, and the bike always had an extra swing of its own when straightening up from a turn.
