I also ran into this on TG flying BKK-LAX via NRT. All fare classes showed as zero when searching BKK-LAX, however because both of these legs are able to carry local traffic I think they zero out BKK-LAX, but continue to sell BKK-NRT/KIX, NRT/KIX-LAX. Try searching each individual leg, and you might find it's not as full as it appears.