Body text after latex title page not showing, Rmarkdown

I have successfully been able to add a latex title page to my Rmarkdown file, but the rest of the document doesn't appear. Here is my YAML:

---

output:
  pdf_document:
    includes:
      in_header: title.tex
---

I haven't changed anything else in the Rmarkdown file, it has the standard text starting with: This is an Rmarkdown document etc. I include my title.tex file:

\usepackage{graphicx}
\begin{document}

\begin{titlepage}
    \begin{center}
    
    \includegraphics{UCTLogoLong.jpg} 
        \vspace*{2cm}
        
        \textbf {\Huge Applied Spatial Data Analysis} % MAIN TITLE
        \vspace{0.5cm}
        
        {\LARGE Assignment 1} % SUBTITLE
        \vspace{1cm}
        
        \noindent\makebox[\linewidth]{\rule{\textwidth}{1pt}} 
        
        {\LARGE Geostatistics}
        \noindent\makebox[\linewidth]{\rule{\textwidth}{1pt}} 
        \vspace{1cm}
        
        {\Large NAME }  % AUTHOR
        
        {\Large STUDENT NUMBER}
        \vspace{1cm}
        
        \includegraphics[width=5cm,height=5cm,keepaspectratio]{statslogo.png} 
         
    \vspace{0.2cm}
        
        Department of Statistical Sciences\\
        University of Cape Town\\
        South Africa\\
        \today
        
    \end{center}
\end{titlepage}

\end{document}

I am relatively new to Rmarkdown and have no idea why the standard Rmarkdown text isn't showing in knitted pdf. Any help will be much appreciated.

The problem is that with the \begin{document}...\end{document} you tell latex that it should ignore everything after that. Instead you could issue the content of the title page \AtBeginDocument{...}

title.tex:

\usepackage{graphicx}
\AtBeginDocument{

\begin{titlepage}
    \begin{center}
    
    \includegraphics{example-image-duck} 
        \vspace*{2cm}
        
        \textbf {\Huge Applied Spatial Data Analysis} % MAIN TITLE
        \vspace{0.5cm}
        
        {\LARGE Assignment 1} % SUBTITLE
        \vspace{1cm}
        
        \noindent\makebox[\linewidth]{\rule{\textwidth}{1pt}} 
        
        {\LARGE Geostatistics}
        \noindent\makebox[\linewidth]{\rule{\textwidth}{1pt}} 
        \vspace{1cm}
        
        {\Large John Doe }  % AUTHOR
        
        {\Large something}
        \vspace{1cm}
        
        \includegraphics[width=5cm,height=5cm,keepaspectratio]{example-image-duck} 
         
    \vspace{0.2cm}
        
        Department of Statistical Sciences\\
        University of Cape Town\\
        South Africa\\
        \today
        
    \end{center}
\end{titlepage}
\clearpage
}

Furthermore, I had to remove the empty line in

---
output:
  pdf_document:
    includes:
      in_header: title.tex
---