Why was a 3D Secure 2 payment authorised with transStatus "A: Attempted to Authenticate from the ARes and without a challenge"?

What transStatus: A Means

When you receive this status, it indicates that:

1. Authentication Was Requested: You, the business, initiated a 3D Secure authentication request for a transaction.
2. Issuer Could Not Perform Strong Authentication: The cardholder's bank (the issuer) received the request but was unable or not ready to perform a strong customer authentication (SCA), such as sending a one-time password (OTP) or requiring a password.
3. Scheme Stepped In: The card scheme (e.g., Visa, Mastercard) intervened and provided an "attempted" authentication response. This essentially vouches for the transaction attempt.

This is different from a transStatus: Y (successful authentication) or transStatus: N (failed authentication). It occupies a middle ground where the full security challenge wasn't completed, but the attempt itself was validated by the scheme.
This comes with Liability shift, the same way a successful authentication would.