HBO fares are non combinable, and that's a system block. Unless a travel agent connects them "illegally" there is no way the connecting flight can have longhaul to shorthaul HBO. On Amadeus I can't see how it can happen, and there are plenty of other non combinable fares that do not cause similar oddities to this. On the other hand, I can imagine some underlying data error here, so I would raise an issue on the BA.com fault thread.

HBO cannot be combined with any other fares at all, other than a return leg also in HBO, from the same location.