An expired certificate has led to the removal of linked American Express credit cards from users’ Google Pay accounts. Google Pay users with linked Amex cards began receiving emails that Google removed their linked card. Users found that their credit cards would be unlinked again in a few hours. Google said an expired certificate is causing Google Pay to remove the American Express cards and that they are working on a fix. American Express told BleepingComputer that the issue has been resolved and customers can add their cards back to Google Pay.
Source: https://www.bleepingcomputer.com/news/google/amex-cards-removed-from-google-pay-due-to-expired-certificate/