This can be common when driving into the sun and the angle is low. The ACC uses a camera and it can be blinded so it disengages. My commute is East in the morning when the sun is coming up and West in the evening when the sun is going down. At certain times of the year when the angles are just right, the adaptive cruise control is useless so I just turn off the "adaptive" function for those times when it doesn't function properly and use it as a standard cruise control. It is annoying and the camera technology is not the best for this reason but other technologies have their weak points as well. Other than turning off the "adaptive" function, I don't know of any fixes.