Force update of tarball flake input

I have the following flake tarball input

inputs = {
    code = {
      url = "";
      type = "tarball";
      flake = false;

And I want to update it, but nix flake update seems to just skip it.
Running nix flake lock --update-input code returns instantly, without apparently doing anything. It doesn’t seem to re-fetch the url of the tarball at all. How does it know that the previously downloaded tarball is up to date?

nix flake lock --update-input code -v --refresh doesn’t improve the situation. I don’t get any additional information in verbose mode.