After taking around 45 RTWs over the last 15 years, I have run into a BA bottleneck. I am booking a 16 segment RTW in A class which includes 7 US segments. The itinerary includes one economy sector, but it isn't the last of the 7. BA are arguing that, as the 7th segment in chronological order is A class, I must pay an additional $500 to fly SAN-LAX. All my previous RTWs have allowed me to elect any of the 7 segments as the designated extra one (at $150 extra) - in this case the 4th segment is economy.

I have done this many, many times in the past, both booking through BA and/or AA. Have the rules changed, or are BA simply wrong?