So, why FADEC accelerates the other engine? Merely because the pilot missed or forgot to put the TL at the proper detent? I´m sorry, but this sounds very buggy to me.
The specifications for thrust control are posted earlier in this thread and *if* that was the cause of the overrun the system performed exactly as the specification said. Software 'bugs' are when software fails to perform as per specification, so it wouldn't have been 'buggy' at all.
Most of the criticism leveled at the AB FBW in the early days was based on the idea that the system would second guess what pilots were doing, so (again, if this turns out to be the cause) in this case the system was doing exactly what the pilots requested. Adding the 'guessing' component would not only draw ire from pilots, but would unnecessarily complicate the software design, making genuine bugs far more likely.