What seems to have happened here is that BA deleted their sector to the ticket, AA then reissued or revalidated the truncated ticket without putting the old sector back, and lo, the trip now starts from London. SABRE is more tolerant of this, it wouildn't work in Amadeus.

It's important, as here to work with airlines and sometimes that's more than one call. If you take unilateral action then it is your own risk. Sometimes it works, sometimes you will be out of pocket. Even on shorthaul trips (and this isn't) the relevant CAA guidance implies that sometimes an overnight stay is unavoidable. When a ticket straddles 2 airlines it's not unusual to have to make a few calls. On IATA terms it is BA's responsibility, however the painful fact of life is that BA and AA are on two very different GDS systems and irrespective of responsibility the main lever sits with AA.

Final point is that a lot of the guidance and advice on ex-EU, B2B and nested tickets in this forum is based on BA plated tickets. Some of it is guided by the UK / EU's regulations in this space. If you buy from AA or even IB then it's by no means certain that the advice here will stand up.