Are you sure you have lane keep assist on and not just road departure mitigation? Maybe its not calibrated right because mine keeps the car dead center when it can see the lines in the road.It really should work as advertised though. My LKAS sometimes shows solid lines (when the lane lines are well marked) but allows the car to wander from the lane anyway, .
That or just suggest they keep it for a few days and periodically test it if the first test drive doesn't show up anything but its also dependent on how good a tech is at picking up on issues, some pick it up right away and others actually need the customer to point out exactly when certain things happen.Maybe record the car drifting off the line and bring it to your dealership's attention? The dealership could help you lodge a complaint with Honda or at the very least people will become aware of the problem on youtube.