The page at https://github.com/actions/cache/blob/main/tips-and-workarounds.md#force-deletion-of-caches-overriding-default-cache-eviction-policy gives an example of deleting a cache created on a feature branch when the PR is merged, to prevent the feature branch's caches from taking up space and potentially evicting the cache from the master/main branch. The example code triggers on:

on:
  pull_request:
    types:
      - closed

and runs against:

...
          REPO=${{ github.repository }}
          BRANCH=${{ github.ref }}

The documentation at https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request suggests that $GITHUB_REF / ${{ github.ref }} should contain refs/pull/:prNumber/merge.

However, in my testing, this is only true if the PR is closed without being merged. In that case, it does, indeed, point to refs/pull/:prNumber/merge. If the PR is merged, however, then the value of ${{ github.ref }} is the target branch, e.g., main. As such, this code will then clear the main branch's cache, which is not only pointless, but harmful!

As such, in our code at icosa-foundation/open-brush#397, I changed it to use ${{ format('refs/pull/{0}/merge', github.event.number) }}. This behaves correct in both the regular close, and close-on-merge cases.

The root cause may a general github actions issue, but until that's confirmed or fixed or clarified, I think the example should be modified accordingly.