<script setup> has different behavior between dev and build

[NeilMcEachin]

Describe the bug

If I define variables in <script setup> they are available inside the options api when working with npm run dev but if I run npm run build and npm run preview those no longer or defined inside options api

Reproduction

https://stackblitz.com/edit/vitejs-vite-k5wxvd?file=src%2FApp.vue&terminal=dev

Steps to reproduce

run project with npm run dev
noticed on mounted count logs 0
notice watchedCount can watch count and is reactive on page

kill server and run npm run build and npm run preview
noticed on mounted count logs undefined
notice watchedCount cannot watch count and remains 0 when button clicked

System Info

This is running inside stackblitz

  System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 16.14.2 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 7.17.0 - /usr/local/bin/npm
  npmPackages:
    @vitejs/plugin-vue: ^3.1.2 => 3.1.2 
    vite: ^3.1.6 => 3.1.8

Used Package Manager

npm

Logs

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Make sure this is a Vite issue and not a framework-specific issue. For example, if it's a Vue SFC related bug, it should likely be reported to vuejs/core instead.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

[sapphi-red]

This is an upstream issue.
vuejs/core#6248