Hi Chris,
This is because the user has a different email configured for merges on GitHub than the one they used for commits in the PR. See the devguide for more information on the topic.
Best way to go forward is to ping the user and ask them to sign it (I went ahead and did this for you). If the backport is important and they don’t respond, then I suggest force-pushing commits with the previous email they used (and signed the CLA for) to the backports.