Bug: cannot update cache in Cargo project

I have made a minimal example of bug.

I have setup a Cargo workspace and want to cache the ~/.cargo and target directories. Calling save explicitly at the end of the script does not overwrite the existing cache after the cache is stored on the first time.

Deleting the previous version of the cache fixes it

Run actions/cache/save@v4
/usr/bin/tar --posix -cf cache.tzst --exclude cache.tzst -P -C /home/runner/work/cargo-ci/cargo-ci --files-from manifest.txt --use-compress-program zstdmt
Failed to save: Unable to reserve cache with key Linux-restore-build, another job may be creating this cache. More details: Cache already exists. Scope: refs/heads/main, Key: Linux-restore-build, Version: cd36736238095b30d245e37cee3eada29643d39d68cc49be99ecf89155bdc73a
Warning: Cache save failed.

The core of the message seems to be another job may be creating this cache. More details: Cache already exists.

The YAML workflow file I am using is

name: Rust test workflow
on:
  push:
    branches:
      - 'main' 
jobs:
  test-restore-build:
    name: Test restore compile and store
    runs-on: ubuntu-24.04
    steps:
      - uses: actions/checkout@v4

      - name: Update APT package index
        run: sudo apt-get update
        
      - name: Install and download required APT packages
        run: sudo apt-get install -y rustup

      - name: Restore Cargo cache
        uses: actions/cache/restore@v4
        with:
          path: |
            ~/.cargo
            target
          key: ${{ runner.os }}-restore-build

      - name: Write to target
        run: cargo build

      - name: Save Cargo cache
        uses: actions/cache/save@v4
        with:
          path: |
            ~/.cargo
            target
          key: ${{ runner.os }}-restore-build

Start by running the linked minimal Cargo workspace and reviewing the provided workflow, especially the actions/cache/restore and actions/cache/save steps and their log output. Trace why saving with the existing key is rejected; done means a subsequent workflow run can update the Cargo and target cache without requiring manual deletion.