Line chomping not working as expected

[mortocks]

Trying to use the action to update a csv file but line breaks don't seem to be working with YAML block chomping (keep).

Example below reads a new issue and adds to a csv.

on:
  issues:
    types:
      - opened

jobs:
  label_issue:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
        name: Checkout
      - name: Append file
        uses: DamianReeves/write-file-action@master
        with:
          path: data.csv
          contents: |+
            ${{ github.event.issue.title }},${{ github.event.issue.body }}
              
          write-mode: append
      - name: Commit & Push
        uses: stefanzweifel/git-auto-commit-action@v4
        with:
          # github_token: ${{ secrets.GITHUB_TOKEN }}
          file_pattern: 'data.csv'
          branch: main
          commit_message: 'Overwritten by Github Actions - ${date}'

I tried using https://yaml-online-parser.appspot.com to parse the yaml to check the block chomping |+ setting and it appears to be correct (replacing GitHub vars with cell1,cell2,cell3 e.g

          contents: |+
            cell1,cell2,cell3
              
          write-mode: append

The output includes multiple \n\n at the end of the string as expected but when running within the action, it just appends the last line without line breaks.

Also strange, it appears to work differently on the first run - hope this makes sense.

Step 1
Start data.csv with single line for columns (no line breaks)

Heading1,Heading2,Heading3

Step 2
Run the action. Result is

Heading1,Heading2,Heading3
cell1,cell2,cell3

Works perfectly. Exactly what I'm after

Step3
Run action again, Result is

Heading1,Heading2,Heading3
cell1,cell2,cell3,cell1,cell2,cell3

The second time around, the |+ setting doesn't seem respected and the last line does not include line breaks

I think my YAML is formatted correctly but happy to be told I'm making YAML mistake.