Server Side Encryption with AWS KMS managed keys require AWS SigVer4, which I am

Question

I am trying to download a file from an S3 bucket, but am seeing the following error:

 api error InvalidArgument: Requests specifying Server Side Encryption with AWS KMS managed keys require AWS Signature Version 4.

However, when creating my S3 Client for the downloader, I am setting this to s3v4. Here is my code:

var getFileContent = func(ctx context.Context, s3Details S3Details, key string) (*manager.WriteAtBuffer, error) {
    client := getS3Client(s3Details)

    head, headerr := client.HeadObject(ctx, &s3.HeadObjectInput{Bucket: &s3Details.Bucket, Key: &key})
    if headerr != nil {
        return nil, headerr
    }

    buff := manager.NewWriteAtBuffer(make([]byte, 0, head.ContentLength))

    _, err := manager.NewDownloader(client).Download(ctx, buff, &s3.GetObjectInput{
        Bucket: aws.String(s3Details.Bucket),
        Key:    aws.String(key),
    })

    return buff, err
}

func getS3Client(s3Details S3Details) *s3.Client {
    endpointResolver := aws.EndpointResolverWithOptionsFunc(func(service, region string, options ...interface{}) (aws.Endpoint, error) {
        endpoint := aws.Endpoint{
            PartitionID:   "aws",
            SigningRegion: s3Details.Region,
            SigningMethod: s3Details.SignatureVersion,
        }

        if s3Details.EndpointUrl != "" {
            endpoint.URL = s3Details.EndpointUrl
            return endpoint, nil
        } else {
            return endpoint, &aws.EndpointNotFoundError{}
        }
    })

    cfg, _ := config.LoadDefaultConfig(context.TODO(),
        config.WithEndpointDiscovery(aws.EndpointDiscoveryEnabled),
        config.WithEndpointResolverWithOptions(endpointResolver))

    return s3.NewFromConfig(cfg, func(o *s3.Options) {
        o.Region = s3Details.Region
        o.Credentials = aws.AnonymousCredentials{}
        o.UsePathStyle = true
    })
}

Verified that s3Details.SignatureVersion is definitely set to s3v4. Is there something I'm missing here?

Using aws-sdk-go-v2