I’ve seen similar when looking at BAH bookings via Doha. It usually comes with a warning along the lines of we’ve suggested seats in economy to keep the price down for you.
If you go to the choose alternative flights page, there is usually an option with business all the way on the same flights, but it typically doubles the overall holiday cost.
i had been wondering why it was happening, but given the usual lack of booking class information for BAH, lack of remaining inflexible fares for the second leg is certainly a plausible explanation