This is not actually true. Torsion bars do not lose their spring rate, it's literally a metallurgic property of the metal.
They do "sag" though, which is why the adjustment changes over time. Unlike a leaf or coil spring that compresses or flattens out though the change is in the offset of the hex ends. If you think about the bar "unwinding" over time that is how the sag works. It's not exactly what's happening, but the result is basically the same.
