Update: trying other starting points, and hence other ticketing airlines

BA ticketing (basic itinerary NBO-LHR-LAX-SYD-JNB)
Same symptoms!
Adding LAX-DFW-LAX or SYD-MEL-SYD causes failure
But having LAX-JFK-DFW worked as does having BNE-ADL-SYD
(ticketing url in this case is http://wftc2.e-travel.com)

Edited to add:
CX ticketing (basic itinerary YYZ-xHKG-PER-HKG-XIY,DME-AMM-KRT-AMM-YUL)
Almost the same symptoms!
Adding segments in SWP had the same symptoms (adding PER-BME-PER failed, but adding PER-SYD-BNE worked)
But in Asia adding HKG-PNH-HKG worked!
(and the ticketing url is the same as that for BA)