Description
Hey,
After having faced the same issue as borisveytsman/acmart#436 when writing a paper with \documentclass{IEEEtran}, I decided to see which packages are related to the issue, and found that it's an interaction between hyperref's booktabs feature and tikzexternalize of tikz
MWE
Consider the following MWE (thanks @JasonGross for producing this).
%\PassOptionsToPackage{bookmarks=false}{hyperref} % This fixes the problem
\documentclass{acmart}
\usepackage{tikz}
\usepackage{pgfplots}
\usepgfplotslibrary{external}
\tikzexternalize
\begin{document}
\title{}
\begin{abstract} \end{abstract}
\maketitle
\section{XXXXXXXXX}\subsection{X XXXXXXXXXX XXXXXXX}\subsection{XXXXXXXX XX XXXXXXXXXXXXXXXXX XXXX}\subsection{XXX XXXXXXXX}
\section{XXXXXX XXXXXXXXXX XXX XXXXXXXXX}\subsection{XXX XXXXXXXX XX XXXX XXXXX}
\section{XXX XXXXXXXXX XX X XXXXXXXX}\subsection{XXXXXXXXXXXXXXXX XXXXXXXXXXX XXX XXXXXXXXXX}
\subsection{XXXXXX XXXXXXXXXXXX XXXXXXXX}
\section{XXXXXXX XXXXXXXXXX}\subsection{XXXXXXXX XXXXXXXXXXXX XXXX XX XXXXX}\subsection{XXXXXXX XXXXXXX XX XXXXXXX}
\subsection{XXXXX XXXX XXXX XXXXXXXXXX}\subsection{XXXX XXXXXXXXXX XXXX XXXXXXXX XXXXXXXXXXXXXX}
\subsection{XXXXXXXXXXX XXX XXXXXXXXXXXXX}
\section{XXXXXXXXXX}\subsection{XXXXXXXXXXXXXXX}
\begin{equation}\label{eq:equation}
x+x^2
\end{equation}
\begin{tikzpicture}
\begin{axis}
\addplot [color=black,solid,line width=1.0pt,domain=0:1,samples=11]{x+x^2};
\addlegendentry{Eq. \ref{eq:equation}};
\end{axis}
\end{tikzpicture}
\subsubsection{XXXXXXXXX XXXXX XXXXXXX}\subsubsection{XXXXXXXXXXX XXXXXXXXXXX XX XXXXXXXXXXXXXXX XXXXXXXXX}
\subsubsection{XXXXXXX XXX XXXXXXXXX XXXXXXXXX}\subsubsection{XXXX XXXXXXXXX}\subsection{XXXXXXXXXXXXXXX XXXX XXXXXXXXXXXX}
\section{XXXXXXX XXXX}
\section{XXXXXX XXXX}
\end{document}
Compiling the above document with
docker run --rm -it --volume "`pwd`:/workdir" -w "/workdir" --user `id -u`:`id -g` registry.gitlab.com/islandoftex/images/texlive:TL2021-2022-01-09-04-05 pdflatex -shell-escape test.tex fails, and the ref used inside the tikzpicture is never resolved.
However, when providing \PassOptionsToPackage{bookmarks=false}{hyperref}, and thereby disabling the bookmarks functionality, the code compiles just fine and the ref is resolved properly.
Note that I also found the same clash to happen with \documentclass{IEEEtran}, which is why I guess that it's an interaction related between tikz and hyperref, rather than a problem with \documentclass{acmart}. Related package versions are,
Package: hyperref 2021-06-07 v7.00m
Package: tikz 2021/05/15 v3.1.9a (3.1.9a)
and full reproducibility should be given by the docker image.
Since I don't know the exact interactions which cause the issue and how to best resolve them, I have also crossposted the Issue to [tikz}(https://github.com/pgf-tikz/pgf/issues/1127).
Best,
ORippler