The GITHUB_WORKSPACE is set with an incorrect directory

Associated GitHub Community ticket: https://github.community/t/github-actions-is-setting-github-workspace-incorrectly/132187

The customer has a workflow like as below:

name: CI
env:
  JAVA_VERSION: 11.0.7
  NOW: $(($(date +%s%N)/1000000))
on:
  pull_request:
jobs:
  evaluate-environment:
    name: Look at the environment
    runs-on: [self-hosted, humio, special]
    steps:
      - name: Print github workspace
        run: |
          echo "GITHUB_WORKSPACE = $GITHUB_WORKSPACE"
          echo "github.workspace = ${{ github.workspace }}"
          echo "pr.ref = ${{github.event.pull_request.head.ref}}"
          echo "github.ref = ${{ github.ref }}"
          echo "$GITHUB_CONTEXT"
        env:
          GITHUB_CONTEXT: ${{ toJson(github) }}

  checkout-code:
    name: Check out the code and see what ref comes out
    runs-on: [self-hosted, humio, special]
    steps:
      - name: Remove existing git hooks
        run: git config --unset core.hooksPath && rm -rf .git/hooks/ || true

      - uses: actions/checkout@v2
        with:
          path: ${{ github.ref }}
          clean: true
          lfs: true

      - name: Checkout LFS
        run: git lfs checkout

      - name: Print workspace path
        run: echo $GITHUB_WORKSPACE && pwd && echo ${{github.ref}}

      - name: current checkout
        run: git log -10

When viewing the value of 'GITHUB_WORKSPACE' and 'github.workspace' printed in the logs, he found that the path was "/actions-runner/gh-ci-01/_work/humio/refs/pull/8401/merge".
Normally, the path of the default workspace should like as below:

<runner-installation-directory>/_work/<repo-name>/<repo-name>
OR
<runner-installation-directory>/work/<repo-name>/<repo-name>

In the customer's case, the correct path should be "/actions-runner/gh-ci-01/_work/humio/humio". But why the path was changed?

Start with the self-hosted runner workspace handling and reproduce the supplied workflow, comparing GITHUB_WORKSPACE with github.workspace and the checkout path. Check how actions/checkout@v2 behaves with the pull-request ref and path settings; done means identifying why the workspace resolves to the pull-request merge directory and documenting or correcting the expected behavior.