Codebuild custom cache is emptied mysteriously

I am running CodeBuild for building rust binaries and caching Cargo packages for faster build times using the custom cache feature. Buildspec yaml below. I am running into a problem where the build cache resets approximately every second build.

version: 0.2

phases:
  build:
    commands:
       - PATH=$PATH:$HOME/.cargo/bin && ls $HOME/.cargo/registry/cache/ && PKG_CONFIG_ALLOW_CROSS=1 cargo build -j 8 --target=armv7-unknown-linux-gnueabihf --release && s3cmd put ./target/armv7-unknown-linux-gnueabihf/release/example_* s3://example-product-builds/ --acl-public
cache:
  paths:
     - /root/.cargo/registry/cache/
     - /root/.cargo/registry/index/
     - /root/.cargo/registry/src/
     - ./target/

The cache works, but not on every build. For some reason approximately every second build the cached folders are empty. I have checked this by running ls on the cache directories during the build. Also, there is a significant impact on build time so it is obvious if the cache works or not. For example, the last 10 builds have been 4 minutes without cache and 30 seconds with cache. I have also verified that the build time does not depend on the source code by building the same source multiple times in a row.

I have also tried the /**/* syntax and it has the same problem

cache:
  paths:
     - /root/.cargo/registry/**/*
     - ./target/**/*

Edited by: devaj on Nov 18, 2019 5:20 AM