Probably same/similar issue.

Upcoming return flight - booked on ba.com and both out/return both BA flights/ticketed. Had put all the info in and outward flight okay - so API must have been okay. Now find all my API info has gone for the return - no API information in there at all, so re-entered all the data - no errors, exited, and MMB still says API needed. Tried 3 times - AIP won't keep the information. (Each time on exit it says 'complete', then MMB says it's incomplete - confirmatory email is non-committal!)

Same initial issue with Mrs CKBA, although API had remembered most of her details, and once the 'missing' data had been entered that data stuck. Uhmmmm....