noticed the same/similar issue, me and my wife we both have our profile saved in the website and i'm booking few flights per year always by using the same data collected in the website; which are reporting our correct birthdays. last month i booked a flight and in the confirmation page the site was automatically reporting our birthdays with one day difference.
i called the elite line and thye said they have the right dates in the system and that in any case, birthday is not a relevant data which can be adjusted at any time with no penalties, but it should be solved automatically as might be a system mistake.
so i did the booking regularly
my flight is next month hope nothing strange happens, but while doing online check-in should be able to enter all correct passport info again inc ase of need