Why does the 747-400/400F pull to the left or right after landing?

After performing an autoland you must disconnect the autopilot after slowing down. The PMDG 747 uses the runway's localizer signal to hold the centerline while in ROLLOUT mode. Due to an MSFS limitation, this signal can become eratic further down the runway. The result is that the plane starts pulling to whichever side it thinks it needs to go to to continue following the signal. Once you disengage the AP either by pulling the command bar out or by pressing the yoke switch, the plane should steer normally on the ground.