Plugin files like matomo/plugins/MyPlugin/.hidden do not get copied into the cloned Matomo in https://github.com/matomo-org/github-action-tests/blob/main/scripts/bash/checkout_matomo.sh#L15.

They should be copied, as they might be or may become necessary for automated tests.

(In my case, the file is named .wp-env.json and is needed for a new test.)