Problem with Tikzmark and other Packages

Question

I needed help from Marmot on the arrows of this program because my skills are weak, but I made a few attempts alone and got errors I knew were wrong. Removing the package flexisym did the job. Why? It was apart of a larger template so while I don't use it, it was conflicting with the tikzmark package.

\documentclass{article}
\usepackage{amsmath,mathtools,geometry}
\usepackage{flexisym} % Use % to make the program work properly
\usepackage{tikz}
\usetikzlibrary{tikzmark}

\newbox\aebox
\newcommand\aecomment[2][0pt]{%%
  \begin{lrbox}\aebox
    \begin{minipage}[t]{2.5in}
   \raggedright
  \textcolor{blue}{#2}%%
  \end{minipage}%%
 \end{lrbox}%%
 \raisebox{#1}[\height][0pt]{\usebox{\aebox}}}

\begin{document}
For $f(x)=\dfrac{1}{x}$, we have

\begin{align*}
      f(x+h)&=\dfrac{1}{x+h} \\
 \shortintertext{then}\\
                 f(x+h)-f(x)&=\dfrac{1}{x+h}-\dfrac{1}{x} \\ \\
                        &=\dfrac{1}{x+h} \cdot \textcolor{red} 
   {\dfrac{\tikzmarknode{C}{x}}{x}}-\dfrac{1}{x}\cdot \textcolor{red} 
   {\dfrac{x\tikzmarknode{C'}{+}h}{x+h}} && \aecomment[1.3ex] 
   {\textcolor{red}{\tikzmarknode{A} 
   {}Multiplying by $1$ to get a common denominator}}\\
  \end{align*}

 \begin{tikzpicture}[overlay,remember picture]
 \draw[red,-latex] ([yshift=3pt,xshift=-5pt]A.west) -- ++(-7pt,0pt)-- ++ 
  (0pt,12pt) coordinate (aux)-| (C.north);
  \draw[red,-latex] (aux) -| (C'.north);
  \end{tikzpicture}


 \end{document}

This will output with 2 errors the following:

By removing \usepackage{flexisym} the output properly gives:

I don't know what flexisym is but it was incompatible with the tikzmark program!