[Appendix] Using `appendix-style: plain` does not keep header number

---
title: Test
format: 
  html:
    toc: true
    appendix-style: plain
    number-sections: true
---

# Executive Summary {-}

{{< lipsum 1 >}}

# Introduction

## Text

{{< lipsum 1 >}}

Let's add a footnote^[that would be in same place as appendix]

## Subtitle

{{< lipsum 1 >}}

# Appendix {.appendix}

{{< lipsum 1 >}}

## Appendix Subtitle {.appendix}

{{< lipsum 1 >}}

We could expect Appendix to be in side document but with different style than header 2, especially because of # Appendix {.appendix}

though it is not

Reproduce the issue with the supplied Markdown document and HTML options, then inspect the generated headings around the appendix and compare their numbering and styles. The work is done when an appendix using appendix-style: plain retains the expected appendix header number while preserving the plain style.