The engine has a watercooled turbo that superheats the engine coolant before sending it back to the expansion tank via a 'degas pipe' that was found to have a weak union in it. All of these degas pipes should now have been replaced. But the turbo is also oil cooled and if the oil feed and oil return pipes are blocked with carbonised oil from switching the engine off when the turbo is too hot, the bearing will not get its necessary lubrication and will overheat beyond the capacity of the watercooling to cool it. I can't understand why your car had not been properly fixed. In the circumstances I would demand that the supplying dealer buys it back at current market price.