Equation numbers are shown in reverse parentheses

This problem is due to an incompatibility between bidi and polyglossia (as alluded to in V Karen-Pahlav's answer.) bidi expects the boolean \if@Latin to be set to true for RTL scripts, but polyglossia implements a different way to encode language direction on a language by language basis.

\documentclass{article}
\usepackage{amsmath}
\usepackage{polyglossia}
\setmainlanguage{english}
\setotherlanguage{farsi}
\newfontfamily\farsifont[Script=Arabic]{Scheherazade}
\csname @Latintrue\endcsname

\begin{document}

\begin{equation}
a = b 
\end{equation}

\end{document}