I'm not sure it's a mixed cabin fare. My guess is it's a business through fare LON-SYD, but the required booking class on the MH flight for that fare routing isn't available so the system is automatically booking into Y. It may be coming out cheaper because of YQ rather than fare per se. It's not necessarily 'deliberate' in the sense that BA probably isn't really intending to be offering that specific itinerary, but they need to do some tweaking to surface something better. If OP could post the dates that would help check what's actually been constructed.