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}