There's a difference between top speed on the flat vs top speed going up a hill, the motor is having to work significantly harder in the second scenario.
From the manual:
The electric drive transmission may become very hot in a high-temperature environment with frequent starting, frequent rapid acceleration and deceleration, long-term continuous steep climbing, and overload of the electric drive transmission. In order to prevent damage to the motor, the system will perform an overheating protection function, the warning message “EDU Coolant Overheating”displays. In this case, park your vehicle safely or, while keeping a lowload, continue to drive your vehicle at a constant speed to cool the motor. Only when the motor temperature has decreased, and the warning
message disappears, can the vehicle be driven normally.
I think the problem is either the car isn't reporting the warning correctly as bowfer appears to have it occur when on a hill accelerating, or the system is misreading the temperature and throttling back to its constant rating as a result.