My own A2 will also do this, but I never allow it to. Even if the VGT turbo in the TDI90 allows enough torque to be developed and the engine seems comfortable, low-rev, high-load scenarios are terrible for the engine and gearbox bearings, and are thought to be a major contributor to oil-pump-mechanism wear. As such, the moment that I go below 60mph, I change down to 5th, even if that increases fuel consumption slightly. Long term, I believe this to be the lesser evil.
I agree with your logic Tom and will now follow the same methodology going forward, I guess it’s the equivalent of doing <1200 rpm in top in the Touran, with its additional cylinder - which I would never do either: 1400 is my minimum for that and only on the flat. Any incline or need for acceleration and I change down.
Still, interesting that the car will do it!