Specific math symbols are not showing in output (parenthesis and plus signs)

Question

I have a peculiar problem that I have failed to solve. I have looked at this and have been unable to fix my problem.

Here is an example of my issue:

Everything looks fine, right? However, when I have equations with plus or parenthesis, it shows up like this:

The original code contains $f(x)=ax+b$. I cannot figure out what is going on. I am using the kaobook template, found here. I am new to it, and here are the packages I'm using:

    \documentclass[
    fontsize=10pt, 
    twoside=false,
    numbers=noenddot,
]{kaobook}

\usepackage[english]{babel} 
\usepackage[dvipsnames]{xcolor}

\usepackage{amsfonts}               
\usepackage{amsmath}                
\usepackage{amssymb}                
\usepackage{centernot}          
\usepackage{mathrsfs}           
\usepackage{mathtools}              
\usepackage{multirow}       
\usepackage{tikz-cd}    
\usepackage{gensymb}
\usepackage{enumerate}

 \begin{document}
  $f(x)=ax+b$
 \end{document}