Thanks for the feedback. Two things jump out at me. First, as I said, the front one especially would stay on for longer after I stopped than the company claimed it would, so perhaps that it is indication that it was being "over volted", perhaps by being too close to the magnets. Secondly, yes, there were some pretty low temperatures here in Maine last winter. It was ridden (with the lights operating, of course) on several days down around 0F (-17C), and the bike was stored on my unheated front porch overnight, so storage temperates probably got close to that for extended periods of time, although the capacitors would not have been operating.
The company website states "The light stores the energy in a built-in capacitor and keeps flashing for several minutes while the bicycle is stationary." That's all the detail they provide.