Cache miss for existing keys when merging PRs

I have a workflow that is saving a cached file from the head branch to the base branch on a pull request merge. The file keys exists on the head branch, but when it attempts to find it using a restore-keys basic path, it is failing to hit. I see the key exists in my GitHub cache for the head branch, so not sure why it's showing as missing.

Here is an example workflow:

name: Cache file on base branch
on:
  pull_request:
    types: [closed]

env:
  node-version: 20.18.x

jobs:
  cache_file:
    # this job will only run if the PR has been merged
    if: github.event.pull_request.merged == true
    runs-on: ubuntu-latest
    steps:
      - run: |
          echo PR #${{ github.event.number }} has been merged

      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: ${{ env.node-version }}

      - name: Restore file from head branch

        uses: actions/cache/restore@v4
        with:
          fail-on-cache-miss: true
          path: main.json
          key:  file-${{ github.head_ref }}-${{ github.run_id }}-${{ github.run_attempt }}
          restore-keys: |
            file-${{ github.head_ref }}-
 
      - name: Save file to base ref
        uses: actions/cache/save@v4
        with:
          path: main.json
          key: file-${{ github.base_ref }}-${{ github.run_id }}-${{ github.run_attempt }}

Start by reproducing the shown pull_request closed workflow with actions/cache/restore@v4 and actions/cache/save@v4, using the head and base refs in the cache keys. Check the cache action’s restore scope and restore-keys behavior for merged pull requests. Done means the head-branch cache is reliably restored or the limitation and required workflow change are documented.