If you’re doing a search for a round trip or multi-city award, the search engine doesn’t always return all of the possible routes. That is, if you were searching ORD-SIN // HKG-ORD as a single award, you may find that you get fewer choices than you get from searching ORD-SIN by itself. In those cases, the agents can build awards you can’t build yourself. Buying one-way awards, which is what I recommend for this trip, avoids this issue.

(Now that UA has eliminated award fees, the only reason to book a round-trip is if you’re making use of the excursionist perk to get a free one-way flight between your outbound and return. Since you’ll be on a cruise, I’m guessing that’s not useful.