Is it possible that we're talking about the Road Departure Mitigation (RDM), not Lane Keeping Assistance System (LKAS)? I think the speed range is the same for both (45-90 MPH), but they work differently. RDM is is more passive (can just be left enabled always), only activating when you are going out of lane without signalling and is the one that would be "warning" by vibrating the wheel, showing a message on the screen along with a beep, and sort-of trying to push the wheel in the right direction. The button for it is near your left knee, with a picture of a car crossing a lane. Is it possible it was disabled before and the dealer enabled it?
LKAS you need to actively enable each drive, and it attempts to actively steer to keep the car centered in the lane on its own; it doesn't warn or vibrate the wheel as it does so (it does warn and eventually disable itself if it doesn't sense a bit of resistance from your hands on the wheel). This is the LKAS button on the steering wheel.
The speeds in the manual are approximate, so I definitely wouldn't be surprised to see RDM activate if the speedometer showed 44, or if I was decelerating (so when it started detecting/activating I was going faster). And false activations can happen where the lines or patches in the road aren't as clear. If it is clearly falsing repeatably where it didn't before, it could be a camera alignment issue.