I am having the exact same issue, with the same error messages and reproduction steps. I've checked absolutely everything I can think to check, including directly trying Apple's sample app.
I agree that this seems to be an issue on Apple's side, though when I reached out to Developer Support I just got a generic boilerplate response directing me to the documentation.
I have no idea how to get their attention about this issue but I hope they fix it soon...
Topic:
Privacy & Security
SubTopic:
Sign in with Apple
Tags: