This scenario started at FL390 and lasted until FL230 after much head scratching, I'd expect. The CVR would be very interesting in a training sense.
I TL at idle 1 TL at crz. How did they descend? Ever decreasing circles? Side slip?
How did they expect to make an approach in a controlled manner? Did they consider shutting down the engine? An over speeding engine requires a shut down. Fortunately this fault cured it self, but what if..........
'ground test, found satis' seems a bit glib. Why should it not happen again? or were they restricting the a/c to FL230?
There is more to this, and much to learn; both pilots and engineering: from what has been reported by the thread starter.