Including qmd that uses jupyter, yaml top matter gets dumped into render doc sometimes

Bug description

When including a qmd in another document (outer document). If...

• the included qmd executes python, and
• the included document sets a title in its yaml topmatter, and
• the outer document uses pandoc's ```{=markdown} syntax, and
• the outer document does not set a title in its yaml topmatter

Then the yaml for the included qmd gets dumped into the rendered outer document.

Steps to reproduce

run quarto render outer.qmd --to gfm with the files below

outer.qmd

---
---

```{=markdown}
[![CI](https://github.com/machow/quartodoc/actions/workflows/ci.yml/badge.svg)](https://github.com/machow/quartodoc/actions/workflows/ci.yml)
```

{{< include inner.qmd >}}

inner.qmd

---
title: The inner title
some_yaml_option: 1
jupyter:
  kernelspec: python3
---

```{python}
1 + 1
```

output

# kernelspec: python3

[![CI](https://github.com/machow/quartodoc/actions/workflows/ci.yml/badge.svg)](https://github.com/machow/quartodoc/actions/workflows/ci.yml)

------------------------------------------------------------------------

title: The inner title some_yaml_option: 1 jupyter:

``` python
1 + 1
```

    2

Expected behavior

yaml topmatter not included in output

Actual behavior

yaml topmatter in output (see example for output)

Your environment

Mac OS

Quarto check output

Quarto 1.4.398
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.1.8: OK
      Dart Sass version 1.55.0: OK
      Deno version 1.33.4: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.4.398
      Path: /Applications/quarto/bin

[✓] Checking tools....................OK
      TinyTeX: (not installed)
      Chromium: (not installed)

[✓] Checking LaTeX....................OK
      Tex:  (not detected)

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.9.5
      Path: /Users/machow/.pyenv/versions/3.9.5/bin/python3
      Jupyter: 5.3.1
      Kernels: ..SNIPPED..

[✓] Checking Jupyter engine render....OK

(|) Checking R installation...........R scripting front-end version 4.1.2 (2021-11-01)
[✓] Checking R installation...........(None)

      Unable to locate an installed version of R.
      Install R from https://cloud.r-project.org/

Reproduce the problem with the provided outer.qmd and inner.qmd files by running quarto render outer.qmd --to gfm. Trace the handling of included documents, Python/Jupyter execution, and YAML top matter; done means the rendered outer document no longer contains the inner document's YAML.