Debug logs show hashFiles generates a hash; hash gets truncated when actually saving

I am trying to cache a file with the following step:

      - name: Cache machine stats
        id: cache-stats
        uses: actions/cache@v4
        with:
          path: ${{ github.workspace }}/data/stats.json
          key: hist-stats-${{ hashFiles('**/data/stats.json') }}

I already ran the checkout action, the file exists, and when I run the action with the debug option on, the logs show that the file name should be: hist-stats-b97af91963eaed082bd76ddcb2e2eb7dd314fc74b1a09bc908365a0053c00b5d. However, when the cache is finally uploaded, it says Cache saved with key: hist-stats- (no hash), and when I check the caches, only hist-stats- appears. I can't for the life of me figure out why this would be and would appreciate any pointers.

You can see the PR/code that is triggering the action here: https://github.com/ufs-community/ufs-weather-model/pull/2924

I'm sorry if the issue is due to something obvious--I'm rather new to this, and I'm not seeing anything online that helps/applies to my situation.

Start with the workflow step in the issue body and compare the debug log's generated key with the cache upload message. Review the linked PR #2924 and the provided GitHub Actions run to trace where the hash is lost. Done means the cause is identified and a regression check or documented reproduction confirms the saved cache key includes the expected hash.