Observing the same issue. Even managed to see it happen live on a device on our website. Oddity is that first attempt failed with the same "merchantId=XXXXX not registered for domain" message, then on second attempt on same device and some website just a few minutes apart, it worked without issue. But this behaviour appears rare so far - only on some devices and sometimes the first payment does it error like this...but not always. Only unusual thing on our side is that we swapped acquirers and had to create a new Apple Pay certificate.