I created an account on this forum so that I could let you guys know how I managed to overcome this issue! I tried many different options, such as switching controllers, rebinding keys, enabling and disabling the clutch, and many, many more. Finally I decided that I would force V-sync through the Nvidia control panel, since I have had issues in other games where input was not detected due to the framerate being too high or skipping frames, and voila! Now when I drive sequential-shift cars, the shifts are always spot-on. I play with a Fanatec CSR wheel and use the H-shifter, but TDU intuitively switches to sequential shift for cars that realistically have this functionality. Unfortunately, that's where I ran into trouble since the H-shifter cars had no problems. I love this game and refused to give up on this until I could figure it out, and finally I think I've got it. If anyone else is having this problem, try turning on V-sync and see if that alleviates your problem! Good luck!