Injected $frontmatter.title is not searchable

[ansidev]

Describe the bug

I used the custom function _render to inject the frontmatter.title into the rendered HTML.
Issue: The post title does not appear in the search result.

Reproduction

Reproduction: https://stackblitz.com/edit/vite-gna91h5i?file=docs/example.md

Step to reproduce:

1. Search for VitePress.
2. Expected: See What is VitePress? in the search result.
3. Actual: Only see Powered by VitePress

Expected behavior

The post title should appear in the search result.

System Info

I don't know.

Additional context

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: 20.19.1 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  npmPackages:
    vitepress: ^2.0.0-alpha.12 => 2.0.0-alpha.12

Validations

• Check if you're on the latest VitePress version.
• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.