I have an open-jaw round trip I am trying to book:

2/15 Guam (GUM) to Singapore (SIN) via NRT
3/10 Bangkok (BKK) to Guam (GUM) via NRT

If you book the above as a multi leg reservation, the cheapest UA flights are $4,124 in T/Y on the first leg and Z on the final leg. However, if you book it as two one way tickets, you can get GUM-SIN in B for $606 and BKK-GUM for $1,307 in C.

There are cheaper flights using ANA between Tokyo and SIN/BKK, but we are joining my parents in Tokyo and flying to SIN, and also want to be able to upgrade using miles.

I was trying to get a reasonable fare in economy on the United flights. A simple r/t on the same dates from GUM to SIN is $848 in T/W and to BKK is $930 in T/W. The problem is that W doesn't allow an open jaw. Why wouldn't there be ANY economy fare be available for a o/w BKK-GUM flight or combinable on a half round trip basis, when the flights are relatively wide open five months in advance, with the lower fare codes available for other bookings?

I understand married segment logic and that a fare has to be filed for the city pair in order to be able to book a particular fare class that is available on any particular flight. However, I find this instance bizarre...any ideas on how to get a sub $1,000 price for this itinerary?