CodeBuild LOCAL_CUSTOM_CACHE not caching

For the CodeBuild resource within CloudFormation I have:

      Cache:
        Type: LOCAL
        Modes:
          - LOCAL_CUSTOM_CACHE
          - LOCAL_SOURCE_CACHE

In buildspec.yaml:

version: 0.2

phases:

  install:
    commands:
    - ./install-bazel
    - PATH=$PATH:$HOME/bin

  pre_build:
    commands:
    - mkdir -p .local_cache/bazel/disk
    - mkdir -p .local_cache/bazel/repo
    - ls .local_cache/bazel/disk
    - ls .local_cache/bazel/repo

  build:
    commands:
    - bazel build ... --disk_cache=.local_cache/bazel/disk --repository_cache=.local_cache/bazel/repo
    - bazel test ... --disk_cache=.local_cache/bazel/disk --repository_cache=.local_cache/bazel/repo

  post_build:
    commands:
    - ls .local_cache/bazel/disk
    - ls .local_cache/bazel/repo

artifacts:

  files:
  - '**/*'

cache:

  paths:
  - '.local_cache/**/*'

The ls in the post_build phase shows files in those cached directories. The pre_build phase shows zero files in those cached directories on subsequent builds.