Unexpected cache miss

I'm seeing unexpected cache misses when pushing commits to a feature branch. I have the cache action set up like this:

- name: Fetch cache
        uses: actions/cache@v4
        with:
          key: md-deps-cache-${{ hashFiles('mix.lock', '.tool-versions') }}
          path: |
            _build
            deps

When I trigger the workflow by pushing a commit to my feature branch, I see that a cache entry is created with the key md-deps-cache-9e2b2c8317020aef62614eb08fbfe828891380c4accb7ce17f872ae7c8716009

When pushing a subsequent commit, the workflow reports a cache miss, but the cache key is the same as the one created in the previous step: Cache not found for input keys: md-deps-cache-9e2b2c8317020aef62614eb08fbfe828891380c4accb7ce17f872ae7c8716009

This should be a cache hit, but it's not. Oddly, in the Actions -> Management -> Caches, I see "Last Used: 1m ago", which lines up with my second commit push. So it's as if the caches tab thinks the cache has been used, but my workflow reports a cache miss.

Start with the workflow's actions/cache@v4 step and compare its logs with the Actions → Management → Caches entry for the repeated key. Reproduce the second feature-branch push, then determine why the identical key is reported as a miss; done means the cache hit behavior is explained and the required configuration or confirmed defect is documented.