Text vanishes in Gitbook using Bookdown

Iw as working on a gitbook using bookdown in R several months ago. Today, I resumed my work again, but when I rendered the book the font of the output changed dramatically. It vanishes! I don't remember having changed anything on the TAML of the book. This is how it looks now. Right after the word WELCOME, the text becomes lighter and lighter until it literally disappears. It has a sort of vanishing effect that I have no idea how I got.

Thank you for your help.

Best,

this is my _output.yml

bookdown::gitbook:
  config:
    toc:
      collapse: section
      scroll_highlight: yes
      before: |
        <li><a href="./">PovcalNet Internal Guidelines</a></li>
      after: |
        <li><a href="https://github.com/xxxxx/Povcalnet_internal_guidelines" target="blank">Published with bookdown</a></li>
    download: ["pdf", "epub"]
    toolbar:
      position: fixed
    edit : null
    search: yes
    fontsettings:
      theme: white
      family: sans
      size: 2
    sharing:
      facebook: no
      github: no
      twitter: no
      linkedin: no
      weibo: no
      instapaper: no
      vk: no
    info: yes

bookdown::pdf_book:
  includes:
    in_header: preamble.tex
  latex_engine: xelatex
  citation_package: natbib
  keep_tex: yes
bookdown::epub_book:
  stylesheet: style.css

And this is my _bookdown.yml

book_filename: "Povcalnet_internal_guidelines"
repo: https://github.com/xxxxxx/Povcalnet_internal_guidelines/
output_dir: "docs"
delete_merged_file: true
language:
  label:
    fig: "Figure "
    tab: "Table "
  ui:
    edit: "Edit"
    chapter_name: "Chapter "

rmd_files: [
  "index.Rmd",
  "intro.Rmd",

  "Folder_structure.Rmd",
  "Collaboration_in_Git.Rmd",
  "DM_Group_data.Rmd",

  "Handover.Rmd",
  "references.Rmd",
]

before_chapter_script: "_common.R"

And, I am rendering the book using the following instruction,

bookdown::render_book("index.Rmd", "bookdown::gitbook")

I found the problem. Basically, in one of my .Rmd files I create a diagram using the DiagrammeR package. When one of my colleagues removed the diagram, the issue was gone, but we had no diagram. However, I created a separate .mmd (mermaid) file and updated both packages glue and DiagrammeR. All of this solved the problem.

thanks.