Only noticeable engine brake at rpm 1200 follow by rpm surge

My car is a Nissan Teana J32 year 2008 2 L CVT .

Whenever I release the accelerator to slow down the car from cruise speed approx 80 km/h ( without pressing any brake ) . Once the rpm gradually reduce and hit 1200 , there will be a significant engine brake follow by a slight rpm surge back to 1200 and the rpm 1200 will stuck there and kept pushing the car forward until I apply the brake to stop the car . Idle rpm at park mode is very good . No hesitation during acceleration . Fuel consumption is 1.5 km higher than before this problem arises . There is no engine light on in dash board . No OBD 2 fault code appear after scan . Nissan authorized workshop can’t even / not willing to diagnose the root cause .

Appreciate yr advise on the root cause . Thanks .