When you externalize a library, Webpack does not preserve the export in the ESM output.

Given a source (utilities.js):

export * from '@fluentui/utilities';

Run webpack which outputs esm and externalizes @fluentui/utilities:

yarn bundle

Expected output (dist/utilities.js):

export * from '@fluentui/utilities';

Resulted output:

import*as e from"@fluentui/utilities";var r={d:(e,t)=>{for(var o in t)r.o(t,o)&&!r.o(e,o)&&Object.defineProperty(e,o,{enumerable:!0,get:t[o]})},o:(e,r)=>Object.prototype.hasOwnProperty.call(e,r)};r.d({},{});

The repo also has an index.js file which exports * from utilities.js. If this is used as the entry point in the webpack.config.js, the expected/resulted output are the same. (ESBuild has a bug in this situation where the entry doesn't export the external library from the entrypoint which hasn't been resolved for some time. evanw/esbuild#1737)