Can't find an existing thread - sorry if just a bad search on my part.

I was just looking at some trip options and was looking at a standard award (not saver) in Business NA-Europe. On the trip display it showed a number of options with Mixed Cabin where the domestic leg was showing booking into Economy even though the international leg was booking into BusinessFirst. There was First class space on the domestic legs but the display was claiming not available. In my previous experience international business always booked to domestic first and on a standard award that space isn't constrained availability. If you try the same booking as a revenue ticket it books correctly to domestic first and international business. Anybody know what is going on here? What fare class could it possibly be looking for on the domestic legs?