Happened in #175128 (comment)

The documentation says:

When the backport pull request is merged and you have the necessary privileges you can also replace the label 9.needs: port to stable with 8.has: port to stable on the original pull request.

https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md#backporting-changes

It does not make sense to create a backport PR AFTER the backport pr is merged!