Have just seen a shape like that originally discussed; I've seen them before and it was just a matter of time, but so nice and promptly ...... !
Must have been directly overhead Heathrow, at crusing altitude, headed southwest, about 1230 local today. And as before when I have noticed this, as it was laid out from the aircraft it was perfectly normal and straight, but then got distorted into the discussed sinusoidal shape. It then vapourised quite quickly after doing that, breaking up into separate sections like bananas in line astern first.
So it seems it is not a feature of the aircraft, but of the air currents afterwards.