I think it's realistic, the cruise control system only used the electric brakes, like AFB, if it's not necessary you will have to use the air brakes yourself, or slow down ahead of the gradients. The fact that cruise control is not 100% effective is why the only reason LGV is not incredibly boring in my opinion
This is not a bug. The reason you are speeding is because the brakes that the cruise control is relying on is the master controller. The master controller's brakes are far weaker than the train brakes. Now on a level route like Koln-Aachen for example, this would not be an issue. However, the TGV line is full of steep gradients. Anything over a 1.5-2.0% gradient is too steep for the max braking of the master controller to be able to slow the train down. When going downhill, you need to use the train brake as well to regulate the train's speed.
