Submodule cloning: Needed a single revision

Problem:
Pipeline failing to clone submodule.

Environment:
Both the main repository and submodule are private repositories belonging to the same organisation. I am the owner of the organisation and I have created a PAT with full repo access. This PAT is then added as secret the the main repository.

Pipeline Config:

build:

    runs-on: self-hosted

    strategy:
      matrix:
        node-version: [10.x, 12.x, 14.x, 15.x]
        # See supported Node.js release schedule at https://nodejs.org/en/about/releases/

    steps:
    - name: Checkout
      uses: actions/checkout@v2
      with:
        token: ${{ secrets.CI_PAT }}
        submodules: true
        persist-credentials: true
    - name: Use Node.js ${{ matrix.node-version }}
      uses: actions/setup-node@v1
      with:
        node-version: ${{ matrix.node-version }}
    - run: npm ci
    - run: npm run build --if-present
    - run: npm test

.gitmodules:

[submodule "src/shared"]
	path = src/shared
	url = https://github.com/<org>/<sub-module-repo>.git
	branch = main

If I omit the branch it does not work either.

Self-Hosted:
I am running the pipelines on two of our servers. Both have docker installed and I dont have issues with any other repository that does not include any submodules.
They are both running latest ubtunu 20.04 LTS on x64.
Latest git version has been installed using apt install git

Start with the workflow's actions/checkout@v2 step and the .gitmodules entry for src/shared, then reproduce the private-submodule failure on the self-hosted setup described. Compare checkout behavior with and without the branch setting and the supplied token. Done means identifying the cause and documenting a verified configuration or confirmed limitation.