As I understand it the other car came from a 90 degree angle and was not ahead of you, sensor may not have had enough time to pick up on it, or it thought it was a solid object (like a wall).
The front assist has 2 modes:
City emergency braking which is active below 30km/h, so approx. 20m/h, this is designed to reduce the impact speed, not necessarily to stop you from having an accident.
Above that speed it gives you a visual and audible warning (and applying the brakes slightly so the pads are on the discs), if the driver does not do anything this is followed by applying the brakes in such a way that you wouldn't crash (if possible), if an accident is imminent it will slam the brakes on to reduce impact speed.