When applying VEHICLE_FLAG_CAMERA_DECOUPLED to any object, the camera is still locked to the vehicle rotation. For most vehicles, this isn't an issue and the coupled camera works as documented. However, for vehicles with high velocities, turn radius, or are intended to be driven over rough terrain, this makes them extremely nauseating and in some cases borderline impossible to control.
A fix for this feature, or an alternative solution, should be considered.