Place an appendix section after references won't work

[tfran1971]

Tried to modify an Rmarkdown template. I can not get appendix to be placed after reference section. I did the trick as suggested in section 4.5.4 of the bookdown user guide, but does not work.

---
output:
  bookdown::pdf_book:
    fig_caption: yes
    number_sections: yes
    latex_engine: xelatex
    keep_tex: yes
    toc: false
    citation_package: biblatex
documentclass: scrartcl
classoption: abstract=true,
              titlepage=true,
              fontsize=12pt, 
              pagesize=auto, 
              DIV=13, BCOR=5mm, 
              headings=normal,
              bibliography=totocnumbered,
              numbers=noenddot,
              parskip=half,
              bookmarksnumbered
bibliography: library.bib
csl: the-new-england-journal-of-medicine.csl
urlcolor: black
linkcolor: blue
citecolor: blue
link-citations: true
header-includes:
- \usepackage[times]{fontsetup}
    \setsansfont{Arial}
    \setmonofont{Courier New}
- \usepackage{placeins}
- \usepackage{fancyhdr}
- \usepackage{setspace}
- \usepackage{chngcntr}
- \usepackage{microtype}
- \onehalfspacing
- \counterwithin{figure}{section}
- \counterwithin{table}{section}
- \hypersetup{bookmarksopen = false} 
- \usepackage[nameinlink, noabbrev,capitalize]{cleveref}
---

---
nocite: | 
  @Example1999, @Example2000
...
```
{r global_options, include=FALSE}
knitr::opts_chunk$set(fig.path = 'figures/',
                      echo = FALSE, warning = FALSE, message = FALSE)
```

\pagenumbering{gobble}
```
{r child = 'titlepage.Rmd'}
```

\newpage


\FloatBarrier
\newpage


# References

<div id="refs"></div>

# Appendix

[cderv]

Hi,

Can you reformat you code so that we can more easily use it ? https://yihui.org/issue/#please-format-your-issue-correctly