Dart Signature V4 client Error: The request signature we calculated does not match the signature you provided

Question

I am using the following code to connect AWS REST API using IAM authentication. It's working fine on Google Chrome but giving errors in Firefox.

import 'package:aws_signature_v4/aws_signature_v4.dart';
const signer = AWSSigV4Signer();
    final scope = AWSCredentialScope(
      region: 'ap-south-1',
      service: AWSService.apiGatewayManagementApi,
    );
final request = AWSHttpRequest(
      method: AWSHttpMethod.post,
      uri: Uri.https('<URL>', 'PATH'),
      headers: const {
        AWSHeaders.contentType: 'application/json;charset=utf-8',
      },
      body: json.encode(payload).codeUnits,
    );

    // Sign and send the HTTP request
    final signedRequest = await signer.sign(
      request,
      credentialScope: scope,
    );

    final resp = await signedRequest.send();

Error:

{"message":"The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.

The Canonical String for this request should have been
'POST
<PATH>
content-type:application/json;charset=utf-8
host: <URL>
x-amz-content-sha256: <Key>
x-amz-date:20221017T065918Z
x-amz-user-agent:aws-sigv4-dart/0.2.2

content-type;host;x-amz-content-sha256;x-amz-date;x-amz-user-agent
<Key-1>

The String-to-Sign should have been
'AWS4-HMAC-SHA256
20221017T065918Z
20221017/ap-south-1/execute-api/aws4_request
<Key-2>
"
}

I am using docs from https://docs.amplify.aws/lib/project-setup/escape-hatch/q/platform/flutter/

I seen couple of question on similar issue with no answers. Appreciate any help on this.