Next.js chunks and ssgManifest buildManifest blocking render sometimes

Question

I'm using self hosted Next.js v12.1 and couldn't figure out why sometimes the nextjs chunks and ssgManifest and buildManifest show in a webpage test as render blocking scripts. Then sometimes it doesn't. On mobile I'm seeing almost every webpage test its blocking.

Any ideas why these scripts sometimes block render and sometimes don't? Anyway to make the non-blocking case constant?

Here are my scripts of interest:

_next/static/chunks/main-fb95403.js
_next/static/chunks/framework-39ce2d5.js
_next/static/chunks/29107295-1494f23.js
_next/static/chunks/pages/_app-aa9b9b5.js
_next/static/chunks/webpack-4135c46.js
_next/static/chunks/7d4a1fc8-5761f94.js
_next/static/chunks/68720aa6-e7ea202.js
_next/static/chunks/638-3dce4b4.js
_next/static/chunks/187-f856d9f.js
_next/static/chunks/pages/%5B%5B...slug%5D%5D-11483ee.js
_next/static/VAOqpdvYg-1xlcQ8QG1XS/_buildManifest.js
_next/static/VAOqpdvYg-1xlcQ8QG1XS/_ssgManifest.js
_next/static/VAOqpdvYg-1xlcQ8QG1XS/_middlewareManifest.js

Here is example where it doesn't block:

Example where it is render blocking scripts

Answer 1

Turned out to be a bug on webpagetest.