I've been sending a customer access token in the X-Shopify-Customer-Access-Token header, to authenticate the user in the checkout experience. But at least since today, and maybe before, it appears to have stopped working. I'm redirected to the online store's password page (which never used to happen in the past), and even if I log in there I'm still shown an unauthenticated checkout page.
My last changes to this were a month ago, and those were tested and working. It definitely worked one week ago, so it seems to be a very recent change.