Others will know the detailed rules but it sounds as though you have made it think you are backtracking in Asia-Pac. Sometimes the tool gets it wrong (actually there are two tools and they seem to calculate differently). Also, rather than go back to LHR have you thought of going OSL-HEL-SIN instead (on Finnair)? It's a nice airline, great airport and quite a convenient route.

You may also be hitting some availability constraints on those sectors. HKG and SYD have some bottlenecks at the moment as we found on our booking. I usually take a subscription to Expertflyer for a month or two while I'm setting up an RTW, and use it to check for D class (or whatever you are flying in) on specific flights before asking the tool to price the route and again before calling to book.