fast-csv append writing headers to last row

I'm appending data to a CSV file from a JSON rest API call. However, I'm finding whenever I write too it, it's adding the headers to the last item in the list.

My code is as follows

const csv = require('fast-csv')
const fs = require('fs')
const config = require('./config.json');


const date = new Date().getFullYear()
const filename = `${date}${config.filename}`;

async function createOrAppend(data) {
  const fileExists = fs.existsSync(filename);
  const writeStream = fs.createWriteStream(filename, { flags: fileExists ? 'a' : 'w' });
  const csvStream = csv.format({ headers: true });
  csvStream.pipe(writeStream);
  // Write the data array to the CSV stream by looping through each item
  if (fileExists) {
    // If the file exists, we need to skip the header row
    data.forEach(function(item){
      csvStream.write(item, { headers: false });
    })
  } else {
    // If the file does not exist, write the header row
    data.forEach(function(item){
      csvStream.write(item);
    });
  }
  csvStream.end();
  return new Promise((resolve, reject) => {
    writeStream.on('finish', resolve);
    writeStream.on('error', reject);
  });
}

module.exports = {
  createOrAppend
}

With the data coming in formatted as such

  metrics.push({
    region: region.region,
    address: region.name,
    vantagePoint: vantagePoint,
    percentageHealthy: percentageHealthy.toFixed(2),
    dateStart: dateStart.toISOString(),
    dateEnd: dateEnd.toISOString()
  });