Sounds like a known bug. Try this sometime:

Price AAA-BBB ( stopover) + BBB-CCC-BBB (round trip) as a multicity award trip on united.com- - AAA-BBB + BBB-CCC + CCC-BBB

Now price BBB-CCC-BBB (round trip) as a round-trip award on united.com --- BBB to CCC -- same dates.

Very often you'll see a much different set of flights on the BBB-CCC segment; in my experience the selection has been much more limited when priced multicity vs. when priced round-trip. I see this a lot on legal international trips; it's a bit weird to see it on domestic trips.