The creation and maintenance of codeshares is an almost entirely manual process, so the chances are it just hasn’t been created in BA systems yet. They’ll get to it eventually, but who knows whether it’ll be before or after you need to book.

You could ask the BA call centre to raise it, which will work if the agent knows who to pass it to. You could also ask the QR team… it’s very sad, but sometimes a complaint through back channels from a particular airline that happens also to be a major shareholder gets much quicker action than one through internal teams.