How to ignore files changed by another branch merged as the part of updating to latest main

[ewypych]

Hello! I need to track the changes being made to the specific list of files inside the repository, let's name them:

FileA
FileB
FileC

The workflow should run when they are changed (in theory it could run always, but when there are no changed files it's better to not run it), and then do something with them. Here's the fragment of the workflow (a bit anonymised):

on:
  pull_request:
    branches:
      - main
    paths:
      - '**/FileA'
      - '**/FileB'
      - '**/FileC'
jobs:
  my-job:
    name: My job
    runs-on: ubuntu
    steps:
      - uses: actions/checkout@v4
        with:
          fetch-depth: 2
          ref: ${{ github.event.pull_request.head.sha }}
      - name: Get changed files
        id: get-changed-files
        uses: tj-actions/[email protected]
        with:
          json: true
          write_output_files: true
          since_last_remote_commit: true
          files: |
            **/FileA
            **/FileB
            **/FileC
      - name: Print all changed and modified files from JSON
        run: cat .github/outputs/all_changed_and_modified_files.json
<THE REST OF THE WORKFLOW>

It works perfectly as long as somebody didn't change one of these files as a part of the another branch work, then merge those changes to main, and the branch will be updated with the newest main to keep it up to date. Here's the problematic scenario step by step:

1. The branch feature/1 is created.
2. The changes has been made to FileA.
3. Pull Request feature/1 -> main is created, workflows detects the change in FileA (NO MERGING!).
4. The new branch feature/2 is created off main.
5. The file FileB is changed in `feature/2.
6. Pull Request feature/2 -> main is created, workflows detects the change in FileB.
7. PR feature/2 -> main is merged.
8. The feature branch feature/1 is updated to latest main.
9. Some changes are made to feature/1 and pushed along with the merging commit to already opened PR.
10. The workflow in PR feature/1 -> main detect change in FileB coming from the completely different change.

I'd like to avoid detecting files NOT modified directly by changes made to the branch by Developer. I noticed that no matter what combination of checkout action I use (fetch-deep set to 0, 1, 2, ref like in the code above, or empty) the changed-files action tries to compare the latest commit with the commit BEFORE the merging main into the branch, like here:

some commit messages          d9e6784
Merge main into feature       4e90dc32
another commit                67ac78a

In that case the changed files action will compare:

Retrieving changes between d9e6784 (feature/1) → 67ac78a (feature1)

and takes all files changed as a part of 4e90dc32. Of course it's the merging commit, made by git merge main or GitHub UI/IDE equivalent.

Right now it works also perfectly fine with more than 2 commits, as long as they're a part of normal work with branch.

How to avoid that? What am doing wrong? Unfortunately I can't provide links to full runs, but if you need some more information or should I do some tests, please let me know.