AES256 is an encryption algorithm, whereas AWS KMS is a service that helps you manage encryption keys. When using AWS KMS, AES256 may be the algorithm used to encrypt the data, but KMS handles the key management aspects.
If you are using AES256 as a method of encryption you might get the following error:
Caused by: com.amazonaws.services.s3.model.AmazonS3Exception: Access Denied (Service: Amazon S3; Status Code: 403; Error Code: AccessDenied; Request ID: 7XH8CC0xxxxxx; S3 Extended Request ID: xxxxxx+q/dI3wLV+xxxxxx+psVKQYxrgj8Sh8=)Which can cause Artifactory not able to access information on the S3.