I had this issue for the first time ever and I used to switch between my Canadian and UK passports mid trip with no problem all the time. I normally start with the Canadian passport and switch to the UK one if it makes things easier. As my Canadian passport has less than a year to go and I was flying into the Schengen zone I figured I’d change it to the UK. Ba’s site would not do it and after a second very weird error I decided to leave well enough alone. When I had to contact the Gold line to rebook cancelled flights this week the agent said my passport number was scrolling like 50 times on her screen. She had to put me on hold while she got help to clear it up as she said it was the first time she had ever seen anything like it. I think in future I’ll phone BA to change my API info until this bug is cleared up. The change I attempted was done on the website before I started the trip not on the app.