I used to do lots of these but it's been a while...

Trying to get a DONE4 CPH-LHR-LAX-SYD-LHR-CPH (there will be other stops, but I have simplified it for now) with a stopover in each.

The oneworld tool validates the itinerary but quotes a crazy price. Removing the second LHR stop dramatically reduces the fare. I tried again with FRA instead of CPH and the same thing happened (EUR 14405 vs EUR 7261).

I am staring at "Maximum two stopovers permitted in the continent of origin of which a maximum of one stopover is permitted in each direction in the country of origin." wondering why this doesn't allow me two stopovers in London.

I vaguely remember there was some special difference between AONEx and DONEx where an additional London stopover cost £200 for one but not the other ... but can't find that in the rules.

What have I got wrong? Thanks.