weback/css postcss-merge-rules extracts line-height into a merged rule, but css shorthand in the original rule resets it

Link to the code that reproduces this issue

https://github.com/lab-yue/next-cssnano-merge-rules-bug

To Reproduce

1. see built https://github.com/lab-yue/next-cssnano-merge-rules-bug/blob/main/.next/static/css/0ac7a32f9d4628e6.css has .a, .b { line-height: 22px; } breaks origin css priority in https://github.com/lab-yue/next-cssnano-merge-rules-bug/blob/main/src/styles/repro.css

.b {
  font: inherit;
  font-size: 14px;
  line-height: 22px;
  /* ...other reset properties... */
}

To build again:

pnpm install
pnpm build
pnpm start

Current vs. Expected behavior

Current: new rule created
Expected: .b should kept line-height: 22px; inside.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.5.0: Mon Apr 27 20:41:15 PDT 2026; root:xnu-12377.121.6~2/RELEASE_ARM64_T6041
  Available memory (MB): 65536
  Available CPU cores: 16
Binaries:
  Node: 24.12.0
  npm: 11.11.0
  Yarn: 1.22.22
  pnpm: 11.9.0
Relevant Packages:
  next: 16.2.10 // There is a newer version (16.2.11) available, upgrade recommended! 
  eslint-config-next: N/A
  react: 19.2.4
  react-dom: 19.2.4
  typescript: 5.9.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

CSS

Which stage(s) are affected? (Select all that apply)

next build (local)

Additional context

I think the bug comes form builtin postcss-merge-rules and next/dist/compiled/cssnano-simple

Start with the reproduction repository, especially src/styles/repro.css and the generated .next/static/css/0ac7a32f9d4628e6.css, then trace the next build path through the bundled cssnano-simple and postcss-merge-rules code. Verify the behavior with the provided pnpm install, pnpm build, and pnpm start commands; done means the built output preserves .b’s line-height rather than incorrectly extracting it into a merged rule.