NodeList.forEach polyfill missing; IE 11 breaking

[aaronadamsCA]

I'm using Theme UI, which uses NodeList.forEach, which is unsupported in IE 11.

I believe this should be polyfilled out of the box with Gatsby v3, due to the inclusion of core-js v3; yet I'm seeing a lot of this error from IE 11 users:

Object doesn't support property or method 'forEach'

That would seem to indicate the polyfill (web.dom-collections.for-each) is either missing or broken. You can see for yourself here: https://www.nishe.shop/

The only customizations I have relating to Webpack or Babel are these:

.babelrc.json

{
  "presets": [
    [
      "babel-preset-gatsby",
      {
        "reactRuntime": "automatic",
        "reactImportSource": "theme-ui"
      }
    ]
  ]
}

gatsby-node.js

module.exports = {
  onCreateBabelConfig: ({ actions }) => {
    actions.setBabelPlugin({
      name: "@babel/plugin-transform-react-jsx",
      options: {
        runtime: "automatic",
        importSource: "theme-ui",
      },
    });
  },
};

Gatsby 3.1.1. I'm not sure where to look next. I could drop in a polyfill just for this, but I'm concerned it may point to a larger issue, since as far as I know this should be covered out of the box.