Why my workflow associated to my AWS SFTP Server stops after first step?

Question

I have an SFTP Server in AWS (Transfer Family). I associated a workflow to it.

This workflow has 3 steps :

• call a lambda function for each file uploaded to the SFTP server (in bucket_name_same_as_user_name/unprocessed)
• copy the original file (from bucket_name_same_as_user_name/unprocessed to bucket_name_same_as_user_name/processed)
• delete the original file

The lambda function gets rightfully called but for some reasons, none of the other steps are executed. Why is that ? Did I miss something in my configuration ?

Here's details about this workflow.

• The role:

{  
  "Version": "2012-10-17",  
  "Statement": [  
    {  
      "Effect": "Allow",  
      "Principal": {  
        "Service": "transfer.amazonaws.com"  
      },  
    "Action": "sts:AssumeRole"  
    }  
  ]  
}

• The policy attached to the role:

{  
    "Version": "2012-10-17",  
    "Statement": [  
        {  
            "Sid": "CopyRead",  
            "Effect": "Allow",  
            "Action": [  
                "s3:GetObject"  
            ],  
            "Resource": "arn:aws:s3:::${SourceBucketName}/${ObjectName}"  
        },  
        {  
            "Sid": "CopyWrite",  
            "Effect": "Allow",  
            "Action": [  
                "s3:PutObject"  
            ],  
            "Resource": "arn:aws:s3:::${DestinationBucketName}/${ObjectName}"  
        },  
        {  
            "Sid": "CopyList",  
            "Effect": "Allow",  
            "Action": "s3:ListBucket",  
            "Resource": [  
                "arn:aws:s3:::${SourceBucketName}",  
                "arn:aws:s3:::${DestinationBucketName}"  
            ]  
        },  
        {  
            "Sid": "Delete",  
            "Effect": "Allow",  
            "Action": [  
                "s3:DeleteObject"  
            ],  
            "Resource": "arn:aws:s3:::${BucketName}/${ObjectName}"  
        },  
        {  
            "Sid": "Custom",  
            "Effect": "Allow",  
            "Action": [  
                "lambda:InvokeFunction"  
            ],  
            "Resource": [  
                "arn:aws:lambda:eu-west-3:502802710160:function:test-function"  
            ]  
        }  
    ]  
}

• The configuration of the workflow:

[  
  {  
    "Type": "CUSTOM",  
    "CustomStepDetails": {  
      "Name": "call-test-function",  
      "Target": "arn:aws:lambda:eu-west-3:502800000000:function:test-function",  
      "TimeoutSeconds": 60,  
      "SourceFileLocation": "${original.file}"  
    }  
  },  
  {  
    "Type": "COPY",  
    "CopyStepDetails": {  
      "Name": "copy-file",  
      "DestinationFileLocation": {  
        "S3FileLocation": {  
          "Bucket": "test-bucket",  
          "Key": "${transfer:userName}/processed"  
        }  
      },  
      "OverwriteExisting": "FALSE",  
      "SourceFileLocation": "${original.file}"  
    }  
  },  
  {  
    "Type": "DELETE",  
    "DeleteStepDetails": {  
      "Name": "delete-file",  
      "SourceFileLocation": "${original.file}"  
    }  
  }  
]