Adding vuepress to a project breaks the project's modern build

Bug report

Adding vuepress to an existing project using this guide https://vuepress.vuejs.org/guide/getting-started.html#inside-an-existing-project breaks the existing project's modern mode build.

Version 0.14.1

Steps to reproduce

https://github.com/archSeer/tsmodern

yarn build --modern

Search for arrowTest inside app.*.js (not .map)

[{key:"arrowTest",value:function(){return function(){}}}]

Now remove vuepress from package.json, run yarn install, then yarn build --modern and it will work correctly again (it will build as an arrow function).

What is expected?

The modern build should work fine, and contain =>

The -legacy and regular bundle should be different sizes, and the regular one should contain arrow functions.

What is actually happening?

Files are almost identical apart from the sourcemap.

-rw-r--r--   1 speed  staff   394K Sep 19 18:49 app-legacy.49d50903.js
-rw-r--r--   1 speed  staff   394K Sep 19 18:50 app.b75421a2.js

I also can't find the arrow function used anywhere in the modern bundle:

$ rg "=>" app.b75421a2.js
(empty)

Other relevant information

Your OS: OS X
Node.js version: latest
Is this a global or local install? local
Which package manager did you use for the install? yarn

vuejs / vuepress