I'm the OP here and I don't think you have a problem with the rear springs - they rarely fail. They can rust, break or sag, but it's not a common problem these days.

What you are diagnosing from your post appears to be an issue with the shocks/dampers. It's the shocks that control the springs up and down movement.

I suggest that you either get your VW specialist or a suspension specialist to have a look and test drive the car - you may have faulty KONI Sports. So it could well be a warranty job.

Six years on and my Bilstein B8s and standard springs are still going strong!