From memory this was due to engine vibration in this range. The TPE331 isn't the only engine to suffer this problem, the IAE V2500 (Airbus320) also has a keep out range. With the Airbus the engine computers won't allow the engines to stabilise in the zone which is one of the reasons why the thrust levers are partially advanced to ensure the engines match before setting take off power. If one engine sped ahead while the other one lagged you could be off the runway.