Apple's ASAuthentication's. authorizationController:didCompleteWithAuthorization: works just fine during the Passkey Authentication process. The rawAuthenticationData value attached to the ASAuthorizationPlatformPublicKeyCredentialAssertion credential consists of exactly 37 bytes like the WW3 Section 6.1 specification requires. In the case of Passkey Authentication it is only 37 bytes without either attestedCredentialData nor extensions. But during Passkey Registration process, the ASAuthorizationPlatformPublicKeyCredentialRegistration credential's rawAttestionObject does generate attestedCredentialData. Believe it or not, my error was generated during the creation of the JSON dictionary to be posted during authentication. I mistakenly assigned the Base64URL authenticationString to the signature property and the Base64URL signatureString to the athenticatorData property. Very stupid typo that cost me about two weeks of debugging work..

So here is the answer to my own question... I found it more reliable to use NSURLSessionDownloadTask for the purpose of uploading a file to AWS S3. While I remember reading in the "docs" that one could not use NSURLSessionDataTask when operating in background using BGTaskSchedule. I do not recall reading that one could not use NSURLSessionUploadTask. But, Indeed that seems to be the case.... I hope this helps someone else out there.