README without frontmatter does not render HTML headers (Jekyll 4.1.0)

[p3palazzo]

Describe the bug

Expected behavior: when rendering a README.md without frontmatter, the plugin should use the default.html layout and render a complete index.html with HTML headers

Actual behavior: when rendering a README.md without frontmatter, an index.html snippet is generated, containing only the body content without surrounding HTML tags or headers.

Steps to reproduce the behavior

1. Gemfile:

source "https://rubygems.org"
gem "jekyll", "~> 4.1"
gem "minima"
group :jekyll_plugins do
  gem "jekyll-readme-index"
end

2. _config.yml

theme: minima
readme_index:
  enabled         : true
  remove_originals: true
  with_frontmatter: false
plugins:
- jekyll-readme-index

3. README.md

# Begin with Heading 1, without frontmatter

4. Resulting index.html

<h1>Begin with Heading 1, without frontmatter</h1>

[stale]

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