I just ran into this bug in the booking engine for a multi-city booking for two on K-fares for September. Using the multi-city option, I select the available legs individually that are listed after searching by schedule for the complete trip. Then it takes me through the exact same selection process a second time (weird). After selecting the exact same flights again, I am greeted with

Seats are not available for the selected fare class or higher. Please choose another fare class.

If I use the same fare restriction and I select the same city pairs using the roundtrip search option by schedule, I get the same flights I want just fine.... BTW, those same options only appear if I give it the times of my desired outbound/inbound flights.