Head_ref not found for PR's from forked repositories

[grzegorzkrukowski]

I am trying to combine checkout with changed-files actions to get list of files that are changed in PR posted to our repository.

Currently I have:

- name: Checkout
        uses: actions/checkout@v2
        with:
          fetch-depth: 0
 - name: Get changed files
      id: changed-files
      uses: tj-actions/changed-files@v14.3

The problem is that sometimes it includes changes coming from master branch into the changed files.

You can see runs here:
Wrong:
https://github.com/raycast/extensions/runs/5148147932?check_suite_focus=true#step:3:107

Right:
https://github.com/raycast/extensions/runs/5149182650?check_suite_focus=true#step:3:107

I tried solution described here:
#27 (comment)

but because my PRs are coming from forked repositories it doesn't work as shown here:
https://github.com/raycast/extensions/actions/runs/1833497844/attempts/1

It doesn't detect sha in case I do ref: ${{ github.head_ref }}.

Is there anything I am miss understanding here ? Is it due to PRs coming from forked repositories ?
Is there a way to make it work properly ?