Your car is equipped with a lockup torque converter clutch with computer controlled slip.
Not sure how it is on your car, but under steady road speeds in my C280, there will be about 60 rpms of slip at 35 mph. Gradually, as the road speed increases the amount of slip will decrease. There are about 25 rpms of slip at 55 mph and it is fully locked with no slip by about 65 mph.
Thsi is to isolate any torsional vibrations from the driveline at lower engine and road speeds. The lockup clutch applies very, very smoothly, it will be difficult to see an instant lock and unlock at highway speeds.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record