Thank you for that insight. The ATSB report just says that the auto-pilot disconnected, and the context caused me to interpret that the auto-pilot was unable to deal with the engine configuration. It sounded like the auto-pilot was causing the speed to drop to 1 knot below the desired speed, and was unable to increase the speed so it decided to disconnect (which isn't a happy situation).

From the ATSB report: "The PIC was aware that accurate speed control on final would be important to avoid either an aerodynamic stall condition, or a runway overrun. Consequently, the PIC set the thrust levers for Nos 1 and 4 engines to provide symmetric thrust, and controlled the aircraft’s speed with the thrust from No 3 engine.
The autopilot disconnected a couple of times during the early part of the approach as the speed reduced to 1 kt below the approach speed. The PIC initially acted to reconnect the autopilot but, when it disconnected again at about 1,000 ft, he elected to leave it disconnected and to fly the aircraft manually for the remainder of the approach. Due to the limited landing margin available, the CC reminded the PIC that the landing would have to be conducted with no flare14 and that there would be a slightly higher nose attitude on touchdown.
The flight crew briefed the cabin crew to prepare the cabin for a possible runway overrun and evacuation.
The aircraft touched down at 0346, the nosewheel touched down within about 6 seconds, and the PIC commenced maximum braking and selected reverse thrust on the No 3 engine. The flight crew observed that the deceleration appeared to be ‘slow’ in the initial landing roll, but that with maximum braking and reverse thrust, the aircraft began to slow. The PIC recalled feeling confident that, as the speed approached 60 kts, the aircraft would be able to stop in the remaining runway distance. In consequence, the No 3 engine was gradually moved out of maximum reverse thrust. Manual braking was continued and the aircraft came to a stop about 150 m from the end of the runway."