Buble doesn't seem to be transpiling the js files on build or build:production

My website has been running sage for about a year now, we recently ran into an issue with Javascript not transpiling to es5 via Buble. The rule for .js files should be running this?

        test: /\.js$/,
        exclude: [/node_modules(?![/|\\](bootstrap|foundation-sites))/],
        use: [
          { loader: 'buble-loader', options: { objectAssign: 'Object.assign' } },

As a workaround I’ve added to the script in my package to run buble after the build finishes

"build": "webpack --progress --config resources/assets/build/webpack.config.js && buble dist/scripts/main.js --output dist/scripts/main.js", 

This fix works but it would be better to use webpack to handle transpiling the js files on build.

Any ideas or pointers in a general direction would be amazing!