Engine light

Unfortunately we don't this vehicle in the USA so I don't have specific info about your vehicle. However, this issue sounds like an input sensor to the engine/transmission control module.

The way this determines when to shift is to look at vehicle speed, engine RPM, throttle input, and a variety of other sensors. However, if one of these sensors is not operating properly then the vehicle can down shift at "incorrect" times.

Due to this happening when you are removing the accelerator, I suspect the throttle position sensor could be the issue.

Let's start by checking for codes and seeing if there are any that could point us in a better direction.

The other likely cause is just a shift solenoid issue that when the load is removed it downshifts.

Let me know and we can go from there. Thanks