TFE workspace speculative plan does not start when creating a PR from a forked repository

I have a TFE workspace associated to a GitHub repo and a web hook established between the workspace and repo.

If I create a feature branch local to the original repo and create a PR into the master branch I see a status check run that creates a TF plan in my TFE workspace.

If I create a PR from a forked repository branch into the original repo’s master branch then my status check stays in an Expected - Waiting state.

I’ve checked the web hook history in GitHub and both PRs have sent 200 responses.

In my TFE workspace resource I do have trigger_prefixes setup to be <ROOT_FOLDER>/*/*.py

Is it possible that this is incorrectly setup? Why am I seeing this only happen with a PR from a forked repo but not from a local branch on the original repo?