Is it uncommon for tow ropes to break, rather than
a prescribed weak link. If this was a rare anomaly, is there
some logical reason for it happening?

We used the Tost reel system for many years on 2 tow planes and had the same experience: the ropes always broke near the weak link covering but not at the weak link. I suspect the issue is both the knot in the device and the constant flailing of the device and flexing of the rope near the weak link as it is rewound into the holder. We later abandoned the system.
ROY