Cache volumes keep dependency directories between CI jobs. Until now, only successful jobs on the default branch saved their changes, so pull requests kept starting from whatever the default branch last saved.
With GitHub Actions, every successful job from your repository now saves its changes, including pull requests from the same repository. Pull requests from forks and pull_request_target jobs never save. A job that doesn't change a volume leaves the saved version in place.
To save only from some branches, open Runners → Profiles, edit the profile your workflows use in runs-on:, expand Save cache from, and turn on Limit to specific branches. You can then keep the default branch and add branch names or patterns such as release/*. Profiles that already limit saving to specific branches keep their settings. Buildkite and GitLab CI keep saving from the default branch. See the cache volumes guide for details.