Anchor merge key is reported as "Unused key <<"

[richardkraus-adesso]

Using Anchors in Butane works fine, but when using a merge key a warning at $.passwd.users.1.<<, line 11 col 7: Unused key << is shown when compiling the following config (this also happens for other variants):

podman run --rm -i quay.io/coreos/butane:latest > /dev/null << EOF
variant: flatcar
version: 1.1.0
passwd:
  users:
    - &nologin-user
      name: test1
      uid: 1000
      shell: /usr/bin/nologin
      no_create_home: true
      no_log_init: true
    - << : *nologin-user
      name: test2
      uid: 1001
EOF

This does not break any functionality (at least that Im aware of), but I just find it a little annoying..