Cache not found because action searches for "<key>, <key>", not just "<key>"

Question

I am trying to restore from a cache. My job reads:

  testing-windows:
    strategy:
      matrix:
        example: [Frank2Example1, Frank2Example2, Frank2Example3]
    name: Test windows ${{ matrix.example }}
    runs-on: windows-latest

    steps:
    - name: Checkout repository
      uses: actions/checkout@v2
    - name: Cache Frank!Runner dependencies - build
      uses: actions/cache@v2
      with:
        path: build
        key: ${{ runner.os }}-frank-runner-build
        restore-keys: |
          ${{ runner.os }}-frank-runner-build
    - name: Cache Frank!Runner dependencies - download
      uses: actions/cache@v2
      with:
        path: download
        key: ${{ runner.os }}-frank-runner-download
        restore-keys: |
          ${{ runner.os }}-frank-runner-download
... Other steps ...

Step Cache Frank!Runner dependencies - build does not find its cache. The output reads:

Run actions/cache@v2
  with:
    path: build
    key: Windows-frank-runner-build
    restore-keys: Windows-frank-runner-build
  
Cache not found for input keys: Windows-frank-runner-build, Windows-frank-runner-build

In this message, the key is duplicated.

This issue does not apply to the next step: Cache Frank!Runner dependencies - download. That step can find its cache.

I tried running this job multiple times. It succeeds, but the cache remains unavailable as mentioned.

My full code is available at https://github.com/mhdirkse/frank-runner, commit SHA ad4644d1ab6b7389294a6ad3d2c67c9655517fa6. Did I do something wrong or is this a bug in GitHub actions?

EDIT:

As you see in the text, this failure happens on Windows. I am doing the same under Ubuntu, but then there is no error. Both caches work properly under Linux.