I get that, however, I am not trying to upgrade the CX flight. Just take in its original fare class. I have taken lots of GUFs with codeshare flights (QF, CX) and not had an issue, but maybe they slipped through and this one did not. Though i have some recollection of a rule change, around the same time that BA were able to use GUFs on TA bookings. my last one for instance last year, was two sectors on a ticket that had EK, QF, AA, CX and BA flights under a 125 ticket. If that didn’t raise eyebrows I don’t what would…..

So pre covid, and pre the AA flights ( which incidentally BA forced to be ticketed under their flight number which I believe is against the T&C).