I’ve heard back from Github support. They consider this a feature not a bug.
> This is an intentional change that our team recently deployed and I’m
> sorry to hear that’s causing trouble.
> When a user chooses to squash and merge a PR they are taking authorship
> of the commit. If they were merging the PR and creating a merge commit
> that merge commit would be attributed to them as author, so we wanted
> to be consistent with that hence the change. We are also aware that we
> should be setting the author as a co-author and we can see that’s not happening
> every time.