[JonnyA]

As i recall the heating element wires run top to bottom, so the vertical strips you show are not being heated as much as the surrounding screen, presumably as a result of these sections not working as per the rest of the screen, regardless of the measured current and other parameters.
As the screens are only connected to power in a couple of locations with distribution across the screen being within the laminate layer, I would say that that is sufficent evidence of a manufacturing fault - presuming that Volvo want the screen to clear consistantly across its width.
I would push them to replace the screen again - don't mess about, it won't heal itself and is only likely to degrade further with time.