CircleCI not building pre-existing commit of new PR when "only build open PRs" enabled

CircleCI does not seem to be building pre-existing commit of new PR when “only build open PRs” enabled

We’ve seen it a few times where a commit has been pushed before a PR is raised and is “not run” because the above option is enabled, but it is still not run after the PR has been opened (and detected by Circle, as it it liked to from the Circle GUI).

Is the github “PR created” webhook setup to enable this?