When QR/MH are one of candidates of 1st segment, the online tool seems not to work.

I tried.
(1)
HND-HKG-YVR-LHR-JNB-HKG-HND (all flights can be selected and price quoted).

(2)
Same route with a different origination
JNB-HKG-HND-HKG-YVR-LHR-JNB
Seatching a flight for JNB-HKG doesn't finish.
In (1), the tool shows CX(s direct flight and QR's connecting flight via DOH.

As the previous poster mentioned, 1st segment has warning 2600 about MH/QR starting itinerary restriction.
IMHO, that causes this issue.