Partial match even when only using `key`

I've been using something as simple as:

      - name: Restore Rust
        id: restore
        uses: actions/cache/restore@v3
        with:
          path: ~/.rustup
          key: rust

But for some reason, if the cache doesn't exist, it actually matches on a different key, in this case I've named another cache rustfmt:

2023-04-03T15:07:35.2300547Z ##[group]Run actions/cache/restore@v3
2023-04-03T15:07:35.2300888Z with:
2023-04-03T15:07:35.2301091Z   path: ~/.rustup
2023-04-03T15:07:35.2301275Z   key: rust
2023-04-03T15:07:35.2301523Z   enableCrossOsArchive: false
2023-04-03T15:07:35.2301775Z   fail-on-cache-miss: false
2023-04-03T15:07:35.2301993Z   lookup-only: false
2023-04-03T15:07:35.2302195Z env:
2023-04-03T15:07:35.2302425Z   CARGO_REGISTRIES_CRATES_IO_PROTOCOL: sparse
2023-04-03T15:07:35.2302664Z   CARGO_TERM_COLOR: always
2023-04-03T15:07:35.2302882Z ##[endgroup]
2023-04-03T15:07:36.9535578Z Received 0 of 175967939 (0.0%), 0.0 MBs/sec
2023-04-03T15:07:37.9543626Z Received 88080384 of 175967939 (50.1%), 41.9 MBs/sec
2023-04-03T15:07:38.9676366Z Received 175967939 of 175967939 (100.0%), 55.6 MBs/sec
2023-04-03T15:07:38.9683911Z Cache Size: ~168 MB (175967939 B)
2023-04-03T15:07:38.9973074Z [command]/usr/bin/tar -xf /home/runner/work/_temp/a4330e53-f698-4883-9b37-40e2223e5e9c/cache.tzst -P -C /home/runner/work/despair/despair --use-compress-program unzstd
2023-04-03T15:07:40.1776290Z Cache restored successfully
2023-04-03T15:07:40.5518473Z Cache restored from key: rustfmt

It seems to me that the documentation is pretty clear that key is used for exact matches and restore-keys for partial matches. Furthermore steps.restore.outputs.cache-hit is actually set to true.

Is this a bug, or did I misunderstand the documentation?

Start at the actions/cache/restore@v3 entry point and reproduce the YAML with key: rust alongside an existing rustfmt cache. Check key matching and the cache-hit output against the documented contract. Done means the behavior is explained and the implementation, documentation, or tests are identified as needing correction.