Different behavior of \setlength in preamble in acmart documentclass

[sfzhu93]

\documentclass[acmsmall,review,anonymous,screen]{acmart}
% \documentclass{article}

\newlength{\myproofrightwidth}
\setlength{\myproofrightwidth}{0.3\linewidth} % line 1

\newcommand\rightside[1]
{\penalty-1 \mbox{~}\hfill%
 \begin{minipage}[t]{\myproofrightwidth}%
    by #1%
 \end{minipage}}

\begin{document}

\setlength{\myproofrightwidth}{0.3\linewidth} % line 2: necessary for acmart
left text \rightside{\the\linewidth}

\end{document}

In this code snippet, setting \myproofrightwidth in preamble has a different behavior from other document classes, such as article or ieeetran. If line 2 is commented out, the right side prints 0.0pt. However, this behavior is different from article. In article, the right side correctly prints the line width as a non-zero value. In acmart, you have to set up the length again at the beginning of the document to behave the same as in other document classes.

overleaf document for the above code snippet: https://www.overleaf.com/read/ybmyxnydwjff

This bug makes some of the packages to behave differently, e.g., using the \BY command in pfsteps. The snippet also comes from a use case of pfsteps.