I thing the issue above is that OP doesn't have a LAS-LAX fare. Rather they have an LAS-LHR fare. Which means an ineligible international segment. My guess (it's purely a guess) is that the system is being made to break the fare and upgrade the LAX-LHR fare to one that works with LAS-LAX in F or that it's having problems allocating fare between segments.
Regardless I just did an FCM few days ago on a normal one stop domestic fare and had no issue with pricing. (Both miles and cash amounts were correct) LAX-SEA-AUS. Upgraded LAX-SEA for $79. SEA-AUS was medallion upgrade, LAX-SEA wasn't going to clear as it was last minute purchase with other DM higher on list.