I suspect that the reason for this is that the fare rules for LHR-BKK (or more generally EUR-BKK) used to allow routing via HKG (or KUL). Many of these fares no longer allow this and require the long-haul segment to be direct. The consequence is that instead of having a LHR-BKK reutrn (with the outbound in economy and the return in Business), it now contains three separate fare components with the intra-Asia segment being a separate fare component. The rest would be the effect of fare combination rules, which probably prevent the segment booking in a cheap booking class.