The 6% point is an emergency backup. In those markets where it is allowed, the optional upper limit is allowed, and increases the flexibility of the car, but considering it is an emergency fallback for a city car, where the REx was intended as a crutch to help sell the car to people with range anxiety, it doesn't surprise me that that function was not (easily, anyway) enabled in the coding. One man's opinion...if BMW really wanted a general purpose hybrid, they'd have made the engine larger. It didn't fit the profile or intent of the vehicle.
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)