Do you want to request a feature or report a bug?
I don't think it's a bug, but confusing and inefficient behavior.

What is the current behavior?
When module's package.json contains browser, module & main fields, webpack is bundling browser build by default.

What is the expected behavior?
The key point of the module build is to optimize compiled bundle. And even if a package has hq ES build, it's not being used by default in favor of legacy UMD/IIFE builds. So expected behavior is to use module build by default.