Indeed, this is a more extreme example of married segment fare bucket availability that EK use to construct their fares through for example DXB - every sector on an outbound or inbound part of an itinerary needs to be ticketed in the same fare bucket. I myself have had to just purchase an Y class economy fare (as in full fare) in one direction where the first sector was completely full, but the second sector is empty. In the OP's case, the entire outbound sector will be ticketed in F or A (or Z!) and even though the BP may indicate a business class cabin, the fare paid will be F/A and that will (perhaps after a chat with the supervisor if computer says no) grant FCL access at DXB.