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.