So HND-SFO wouldn't show up on its own, which is kinda expected. I looked at a few other airports that JL fly to in SE Asia. I could find routes through CGK, SGN, HAN, MNL in multi-city function, but just not SIN. I also tried other west coast options to end the trip, like SEA/SAN, exactly the same thing.

Is it possible that only SIN-HND was excluded from marrying to a US flight?