FTPClient.retrieveFile returns false, but file is still downloaded

I'm attempting to retrieve a file from an FTP server, using this code.

private class FtpTask extends AsyncTask<Void, Void, Long> {
    protected Long doInBackground(Void... args) {

        FTPClient FtpClient = new FTPClient();
        int reply;

        try {
            FtpClient.connect(ftpServer);
            FtpClient.login(ftpUser, ftpPass);

            reply = FtpClient.getReplyCode();

            if (FTPReply.isPositiveCompletion(reply)) {
                String remoteFileName = "ACPlus.ZIP";
                String localFile = context.getFilesDir() + "/ACPLUS.ZIP";

                // Delete local file first
                File file = new File(localFile);
                if(file.exists()) {
                    file.delete();
                }

                FtpClient.changeWorkingDirectory("/" + ftpFolder);
                FtpClient.setFileType(FTPClient.BINARY_FILE_TYPE);
                FtpClient.enterLocalPassiveMode();

                OutputStream outputStream = null;

                boolean success = false;
                try {
                    outputStream = new BufferedOutputStream(new FileOutputStream(localFile));
                    success = FtpClient.retrieveFile(remoteFileName, outputStream);

                    reply = FtpClient.getReplyCode();
                }
                finally {
                    if (outputStream != null) {
                        outputStream.close();
                    }
                }

                bConnected = success;
            }
            else
            {
                bConnected = false;
            }
        }
        catch (Exception ex) {
            return null;
        }

        return null;
    }

    protected void onPostExecute(Long result) {
        super.onPostExecute(result);

        if (bConnected == true) {
            // Extract local file
            File file = new File(context.getFilesDir() + "/ACPLUS.ZIP");
            if(file.exists()) {

            }
        }
        else
        {
            AlertDialog.Builder msg = new AlertDialog.Builder(context);
            msg.setTitle("Error");
            msg.setMessage("Connection could not be established!");
            msg.setPositiveButton("OK", new DialogInterface.OnClickListener() {
              @Override
                public void onClick(DialogInterface dialog, int which) {
                    // Do nothing
                }
            });

            AlertDialog dialog = msg.create();
            dialog.show();
        }
    }

As a test, I stepped through the code and after I reached had stepped through

// Delete local file first
   File file = new File(localFile);
   if(file.exists()) {
        file.delete();
    }

I stopped debugging, so I knew that the file had been deleted. Then, I started stepping through again; this time it missed out file.delete(); as the if statement was false, so that proved that the delete file had been completed successfully.

However, when I continued to step through, when I reached the line success = FtpClient.retrieveFile(remoteFileName, outputStream);, success had a value of false, which meant my onPostExecute didn't go through the correct code.

However, when debugging it again, this time it did go through the if statement and deleted the local file, suggesting it had in fact downloaded, despite success = false suggesting otherwise.

Have I misunderstood something, or is this the usual behaviour?

from FTPClient.retrieveFile returns false, but file is still downloaded