This means CI is always running over commits that don't exist anywhere in the repo, and change whenever master changes. If you re-run GHA after master changes, CI is testing over different code.

You can disable this on the checkout action: https://github.com/actions/checkout#checkout-pull-request-head-commit-instead-of-merge-commit

Posting because every senior developer that I know is surprised to learn this, so curious if this is the behaviour people expect.