[Pages] Preview deployments not triggered by PRs from forks

According to https://developers.cloudflare.com/pages/platform/preview-deployments

Every time you open a new pull request on your GitHub repository, Cloudflare Pages will create a unique preview URL, which will stay up-to-date as you continue to push new commits to the branch.

Non-production branch pushes triggers preview deployments as expected. But it’s not the case for PRs from forks. Is this a behavior by design?