Elm VSCode plugin does not format on save

I have copied the following question from the Elm Slack channel for posterity.

I am having trouble getting the Elm VSCode plugin to format on save. I have set the absolute path to my global install of elm-format in settings.json. I have also set format on save for elm:

"[elm]": {
    "editor.formatOnSave": true,
  },

I am on an Ubuntu machine, and format does not happen on save or with Ctrl-Shift-I. Does anyone see something that I might be missing?