BOAC, unlikely to be a bug, just a limit of the design / implementation of the autoflight system. See
http://www.pprune.org/tech-log/40799...ml#post5556610
It’s possible / probable that as the aircraft type has evolved, various ‘short cuts’ have been used in the development and certification.
Thus, when a ‘new autopilot’ is installed, some aspects of the ‘old’ aircraft mathematical model may have been retained, or the 'old' auto system muscle / rate limits retained in the autos design spec. There may be similar ‘adjustments / assumptions’ when adapting an existing auto system to a new aircraft variant, also when adding new FMS derived modes.
FMS modes may actually provide autopilot output demands; alternatively, the FMS and autopilot have to be harmonized, and in many systems, the autothrottle has to be similarly matched, but often only as an after thought.