Version

3.1.1

Reproduction link

https://codesandbox.io/s/busy-kowalevski-0qdy0?file=/src/App.vue

Steps to reproduce

1. Create an SSR App
2. Comment any part of the template code
3. Try to render anything

What is expected?

Hydration should work without errors

What is actually happening?

Hydration fails with the message:

runtime-core.esm-bundler.js:163 [Vue warn]: Hydration children mismatch in <div>: server rendered element contains fewer child nodes than client vdom. 
  at <App>

Can't reproduce and the codesandbox is not editable. Let me know if you change it to reproduce the code and say what template code should be commented.