You could contact oppo service at email@example.com
Or Sales head firstname.lastname@example.org
May be they will be of more help to you.
I just purchased one used from Ebay. It's otherwise in mint condition with an extra batt and extra backplate, so I really can't complain, but it does have the yellowing. It's not really that bad though. Build 6-10-2014 US model X9006_12_140712.
Does it go away after flashing a rom. I intend to head over to a rom on this fella very soon. I would assume not if it's hardware related and most likely is.