Cognito refresh token with secret key causes unable to verify secret hash

Question

I am using below code to refresh token in an AWS Cognito application configured with secret key. No matter which configuration I have tried it always causes common issue of unable to verify secret hash. The input parameters have been trippled checked and the login functionality works well. Can anyone advice on what is the issue with the below codeset.

 public static async Task<AuthFlowResponse> GetRefreshAsync(string userName, string refreshToken)
        {
            AmazonCognitoIdentityProviderClient provider = new AmazonCognitoIdentityProviderClient(new AnonymousAWSCredentials(), FallbackRegionFactory.GetRegionEndpoint());
            CognitoUserPool userPool = new CognitoUserPool(userPoolId, clientId, provider, clientSecret);

            CognitoUser user = new CognitoUser(userName, clientId, userPool, provider, clientSecret);

            user.SessionTokens = new CognitoUserSession(null, null, refreshToken, DateTime.Now, DateTime.Now.AddHours(1));

            InitiateRefreshTokenAuthRequest refreshRequest = new InitiateRefreshTokenAuthRequest()
            {
                AuthFlowType = AuthFlowType.REFRESH_TOKEN_AUTH
            };

            return await user.StartWithRefreshTokenAuthAsync(refreshRequest).ConfigureAwait(false);
        }