`up --frozen-lockfile` doesn't fail when lockfile is outdated

npx @devcontainers/cli up --help says:

  --frozen-lockfile                 Ensure lockfile exists and remains unchanged;
                                     fail otherwise.   [boolean] [default: false]

I just ran up --frozen-lockfile twice, changing devcontainer.json (Really simple - image and feature. Changed feature from one to another.) in between without updating the lockfile. Got success and the same container id both times.