I have this issue with my usual AppleID credentials - they were listed in Sandbox iOS Settings and I successfully performed sandboxed purchase (note - this apple id is not listed in Sandbox Testers list in App Store Connect).
I have no issue (not yet at least) with another dummy AppleID that I have created via Sandbox Testers page.
sashatc
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution
Last seen
User for
Post
Replies
Boosts
Views
Activity
Quite an old question, but I'll try to answer it for others even if OP have already figured it out.
There is no code of requesting the sign-in flow posted, so I'll just assume that OP combined 2 types of request like it is described in Documentation
let requests = [ASAuthorizationAppleIDProvider().createRequest(),
ASAuthorizationPasswordProvider().createRequest()]
Doing this results in no authorizationCode being returned (or actually returned as nil) in ASAuthorizationAppleIDCredential
As for why this happens? Good question I do not know the answer yet.
+1 and yes, I am naive enough to expect it to work in XCode 13 and iOS 15 sim