This is not normal AGS behavior. You should see if it downshifts above the normal downshifts speeds in manual mode too.
The normal upshift/downshift speeds in alto AGS are
* 4--> 5 50 km/h | 5 --> 4: 40 km/h
* 3 --> 4 40 km/h | 4 --> 3 30 km/h
* 2 --> 3 30 Km/h | 3 -->2 20 km/h
* 1 --> 2 ~15 km/h | 2 --> 1 ~5 km/h
Releasing the accelerator in 5th gear at 55 shouldn't lead to a downshift. It should stay in 5th gear, and only on slowing down to 30 km/h should it downshift to 4th gear. Similarly, it shouldn't downshift in other gears either unless at the low-speed limit of the current gear. Only exception to downshifting above the lower speed limit of current gear is when you press the gas pedal and the car downshifts to give you more power.
Might be a problem with a sensor leading to wrong calculation of gearbox input or output shaft speed, or engine load value. What's your car model and mileage? Have you changed the gear oil if over 50-60k driven?