yarn global add breaks offline mirror when run inside a package.json script

[DanielSchaffer]

Do you want to request a feature or report a bug?
Bug

What is the current behavior?
When running a yarn global add command from a package.json script for the first time, it inexplicably deletes many of the packages stored in the offline cache, causing any subsequent offline install to fail.

If the current behavior is a bug, please provide the steps to reproduce.
Repro repo: https://github.com/DanielSchaffer/yarn-offline-global-bug
To reproduce, run yarn cache clean (to ensure simulating a new user setting up their workspace), then run yarn setup.

What is the expected behavior?
Yarn should install the global packages without affecting the local offline cache.

The correct behavior is shown by running the following steps directly at the command line:

yarn cache clean
yarn global add grunt
yarn install --offline

For some reason, yarn global add only breaks things when run in a package.json script.

Please mention your node.js, yarn and operating system version.

• Node.JS: 8.12
• Yarn: 1.12.3
• OS: macOS 10.13.6