Could it be the Mission Editor waypoint type used in the mission? On p. 129 of the User Guide, (see screenshot below), the two types of waypoint related settings are Turning Point and Fly Over Point. The M.E. behavior for Turning Point is that the aircraft will “turn short” of the waypoint so as to roll out on centerline of the next leg segment. The M.E. behavior for Fly Over Point is that the aircraft will fly over the waypoint and then correct back to centerline of the next leg segment.
Whether the DCS F-18 avionics can override these M.E. Settings of the mission builder, I am not sure. Also, Radius of Turn, (ROT), is affected by airspeed. Faster airspeed produces greater ROT. If you’re flying faster into the waypoint, a greater ROT is produced which requires you to turn earlier to roll out on centerline of the next leg segment. Also not sure about aircraft status as AI vs. player controlled. I realize that this discussion is centered on player flown aircraft. Hope that helps. Testing to ensue. :)