`concurrency` with properties now causes "syntax error"

By making a trivial change to an existing test_build.yaml, GitHub Actions complains about a syntax error in the concurrency property.

Invalid workflow file : .github/workflows/test_build.yaml#L3
You have an error in your yaml syntax on line 3

• I’m running in the github.com Actions environment, not my own.
• If I re-order the properties it remains broken.
• If I remove the properties from concurrency it works again.
• I’ve validated that the yaml is valid yaml. So the error is confusing and unhelpful.

Note: This exact yaml file has been working for months. The only change I made was to rename a job.

Broken Example

name: Project CI
concurrency:
  cancel-in-progress: true  # "Syntax error" here?
  group: muffins
on:
  pull_request:
    branches: [devel, staging, prod]
  push:
    branches: [devel, staging, prod]
jobs:
  test_ui:
    defaults:
      run:
        working-directory: ./ui
    runs-on: ubuntu-latest
    steps:
      - name: Checkout Repository
        uses: actions/checkout@v2
      - name: Install Node
        uses: actions/setup-node@v2
        with:
          node-version: 16
      - run: npm ci
      - run: npm run validate

Working Example

name: Project CI
concurrency: muffins
on:
  pull_request:
    branches: [devel, staging, prod]
  push:
    branches: [devel, staging, prod]
jobs:
  test_ui:
    defaults:
      run:
        working-directory: ./ui
    runs-on: ubuntu-latest
    steps:
      - name: Checkout Repository
        uses: actions/checkout@v2
      - name: Install Node
        uses: actions/setup-node@v2
        with:
          node-version: 16
      - run: npm ci
      - run: npm run validate

[mrparkers]

I don’t think I’m the biggest fan of the “silently fix the issue without acknowledging its existence” approach that GitHub seems to have taken here, but I suppose I’m glad that my workflow is working again.

[mrparkers]

I have also posted an issue about this on the community page: https://github.community/t/breaking-change-to-concurrency-group-syntax/215604

A lot of folks appear to be affected by this.