I am trying to book a Skymiles award ticket from SGN - MDC, but the system can't seem to give me the most direct and efficient routing despite availability.

Specifically, on any given date - say, for example, 7/26/18, the search returns two-stop results going through ICN or CAN when a one stop is available. (SGN->CGK->MDC, rather than the returned SGN->ICN->CGK->MDC or SGN->CAN->CGK->MDC).

With VN and GA full Skyteam members, availability on each segment open and bookable, and considerably > MCT (5hrs), shouldn't this book on the most direct and efficient routing?

Editing to add two more datapoints:
-- SGN->DPS produces the same problem, routing 16+hours via ICN or CAN when a much faster, more direct routing is available (VN & GA).
-- Reversing direction has no impact.

What might be the cause and/or fix here? Obviously this is a pretty narrow question but I figure whatever issue is preventing me from booking this itinerary has applicability elsewhere in the program. The DM line and international desk both gave me complete non-answers and weren't able to help at all.

Any illumination greatly appreciated!