As others advised, while counterintuitive, you need to have BKK-SFO (via SIN) available as Saver Award as else the agent constructs your itinerary out of two one ways which each price separately. Don't look for logic in this that a normal passenger could understand.
Now experienced supervisors should be able to address that issue and overwrite the mileage pricing, but finding one willing and able requires patience and a smidge of luck ( well more than a smidge
).