Thanks for the replies, everyone. A few points so far:

- I think there may have been some sort of glitch with the cardmember pricing not factoring in. When I tried to book the one-way at the 406k rate, at checkout it dropped down to 384k with some note saying that the pricing had changed to reflect my dad’s status. My dad recently switched from the Select to Quest card, and I don’t think he’s received all of the new card’s benefits for some reason, so there could be some kinks to sort out here. This explains it better than just “I was searching too much, therefore UA raised the price on me.”
- It’s possible someone may have booked a ticket while I was looking, though I don’t think that’s likely. The seat map shows the J cabin is completely open. (Seat maps, I know, I know…) The flight still shows JN9.
- The one-way fare has gone down to 306k this morning. It still drops to 255k when booked with the DEN-HKG segment.
- Removing the SFO-DEN portion did not affect the price. Neither did adding an intra-Asia segment. Neither did making it a SIN-DEN-SIN roundtrip. It’s only when I add the DEN-HKG outbound does the return fare drop to 255k.

Again, I never thought that one-way award tickets priced differently than roundtrips or multi-city open jaws. Maybe I'll try calling tonight and see if a kind agent might be able or willing to give us the roundtrip pricing but split the booking across two accounts.