The unannounced downgrade/mixed cabin booking is unfortunately not specific to this route nor to this airline. It is an incredibly frequent occurrence on BA bookings that involve any codeshare, regardless of the operating carrier, and regardless of whether the trip is booked as flight only or BAH or the destination.

The message could be clearer,but the reason is in fact that there is no availability in the bucket that you would need for this particular fare, so there would be availability in the cabin per se, but there isn't for the fare that you have selected. It's not the clearest message, but I also struggle to find a way to explain things that would be accurate and short and understandable to most bookers.

In any case, unlike you, I couldn't care less about the explanation given, but what I would actually like is for BA to flash the downgrade a lot more clearly than it does whenever it occurs (or ideally would indicate the cheapest alternative routing that does not require a downgrade) so that customers can decide whether tey are happy with the downgrade based on it being explicitly flagged to them. Going by threads on this forum, we know that people frequently book without noticing it (especially as sometimes, it is only flagged late in the process, and sometimes even only in the confirmation) and that, to me, means that the information is not sufficiently clear to many people, which in itself is very poor practice.