Cache miss for existing cache

I'm getting constant cache misses for caches I can see in the interface.

My cache save step in workflow (a):

      - name: Save Slack thread to cache
        uses: actions/cache/save@v4
        id: slack-thread-id
        with:
          path: /tmp/slack_thread_id
          key: slack-thread-id-${{ github.event.pull_request.number }}

My cache restore step in workflow (b):

      - name: Restore Slack thread ID
        uses: actions/cache/restore@v4
        id: slack-thread-id
        with:
          fail-on-cache-miss: true
          path: /tmp/slack_thread_id
          key: slack-thread-id-${{ github.event.issue.number }}

Both steps report the same key (e.g. slack-thread-id-16), and I can see that cache in the GH web interface. However, I still get Error: Failed to restore cache entry. Exiting as fail-on-cache-miss is set. Input key: slack-thread-id-16.

Start by examining the cache save and restore action entry points used in workflow (a) and workflow (b), especially actions/cache/save@v4 and actions/cache/restore@v4. Reproduce the reported key in both workflows and trace the cache lookup context. Done means identifying why the visible cache is not restored or documenting the required behavior and adding coverage if an existing test area is found.