Encountered a very frustrating booking / ticketing quirk on BA.com.

Trying to book AMS-LAS and take advantage of a low cost uplift to travel in F on the LHR-USA leg. Doing that bumps the two connecting flights (AMS-LHR, USA airport-LAS) down to economy. But booking the whole itinerary in J puts these flights (all flights) in J. Any ideas why and how to fix?

Multi-city search ... using exactly the same itinerary ... returns nothing as usual.