Including markdown prints frontmatter literally

[nimonian]

Describe the bug

When including b.md inside a.md, the frontmatter of b.md is included literally.

Reproduction

Create the file a.md:

---
title: A
---

This is the a.md file

<!--@include: ./b.md-->

Now create the file b.md:

---
title: B
---

This is the b.md file

The resulting HTML looks like this:

<p>This is the a.md file</p>

<h2>
title: B
</h2>

<p>This is the b.md file</p>

Expected behavior

The frontmatter in the included file (b.md) should be ignored.

System Info

System:
    OS: Linux 6.2 Ubuntu 22.04.3 LTS 22.04.3 LTS (Jammy Jellyfish)
    CPU: (8) x64 Intel(R) Core(TM) i7-4980HQ CPU @ 2.80GHz
    Memory: 9.96 GB / 15.49 GB
    Container: Yes
    Shell: 5.1.16 - /bin/bash
  Binaries:
    Node: 20.5.1 - /usr/bin/node
    npm: 9.6.6 - ~/node_modules/.bin/npm
  Browsers:
    Chromium: 119.0.6045.105

Additional context

No response

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