How to fix some compilation issues using R Markdown and Latex markups?

[Rnewuser]

Hi everyone! I've been using Latex for years now and used it for my PHD, but now I've changed my workflow, I'd like to use R Markdown in every file I work on, especially research articles. To this purpose, I've built a YAML preamble and insert some Latex markups in order to output a documentclass book, but I have some complications:

1. I use fontspec and a specific font and when I compile my file, dashes are not displayed between pages numbers while it displays correctly with the standard font. Why? And how to fix that?

2. The TOC displays wrong numbers in the frontmatter as you can see on the picture:
   

   

3. In the backmatter, the header display the previous chapter, not the current one. Why? and how to fix that?
   Thanks for your help.
   Those are my MWE and my preamble.tex:

---
title: A title
date: 
documentclass: book
fontsize: 12pt
geometry: left=6cm,right=6cm,top=7cm,bottom=7cm
linestretch: 1.1
toc-title: Table
links-as-notes: true
link-citations: yes
bibliography: "r-references.bib"
csl: "apa7-fr-couture.csl"
header-includes: 
- \usepackage[french]{babel}
- \usepackage{hyperref}
- \usepackage{fontspec}
- \setmainfont[Numbers=OldStyle,Mapping=tex-text]{Janson Text LT Std}
output:
  pdf_document:
    includes:
      in_header: preamble.tex
    latex_engine: xelatex
    toc: yes
    toc_depth: 1
    number_sections: yes
  html_document:
    toc: yes
    toc_depth: '5'
    df_print: paged
  word_document:
    toc: yes
    toc_depth: 5
---

\frontmatter

# Synopsis

\mainmatter

# A chapter

\backmatter

# A chapter

# References

My preamble.tex :

\frenchspacing

\usepackage{fancyhdr}

\pagestyle{fancy}
% We don’t want chapter and section numbers
\renewcommand{\chaptermark}[1]{\markboth{#1}{}}
\renewcommand{\headrulewidth}{0pt}
\fancyhf{} % sets both header and footer to nothing
\fancyfoot{}
\fancyhead[LE,RO]{\thepage}
\fancyhead[CE]{\textit{A title}} % even pages: chapter title
\fancyhead[CO]{\textit\leftmark} % odd pages: book title


\usepackage{titlesec}
\usepackage{titling}
\titleformat*{\section}{\normalsize\itshape}
\titleformat*{\subsection}{\normalsize\itshape}
\titleformat*{\subsubsection}{\normalsize\itshape}
\titleformat{\chapter}[display]%
    {\centering\Large\bfseries}% format of chapter
    {\large\normalfont\MakeUppercase{\chaptertitlename}\enskip\thechapter}% format of the label
    {.5\baselineskip}{} % space between the two
\titlespacing*{\chapter}{0pt}{25pt}{35pt} % spaces around \chapter{}, {left}{before}{after}
 
%\titleformat{\chapter}[display]
  %{\normalfont\bfseries}{}{0pt}{\Huge}
  
  
  \setlength\parindent{15pt}\setlength{\parskip}{0.0pt plus 1.0pt}

[Rnewuser]

The \frontmatter has to be placed before \maketitle. But, as \maketitle was not inserted into my file, it could not work.