Inspecting the flex-pcb shows the likely fault. Over time the flex-pcb has developed a weak spot, which then becomes the point that flexes the most, causing it to weaken even further.

Flexing the cable shows that this spot is bending at a sharp angle.

Note how the main ribbon splits into two, one side going to the motor and the other to a sensor. The narrower ribbons could also have a fault as they also flex (orange), but in this lens the fault occurred during flexing of the main ribbon (changing zoom as the barrel elements move), not as focus changed. I will only be repairing the main ribbon.