% !Mode:: "TeX:UTF-8:Main"
% to avoid the bug: use  fontloader 2017-02-11-stix or later

\documentclass{article}

\usepackage{unicode-math}
\setmathfont{STIX2Math.otf}

\begin{document}
$\left(\frac{\frac{a}{b}}{\frac{a}{b}}\right)$
\qquad
$a^{\left(\frac{\frac{a}{b}}{\frac{a}{b}}\right)}$ %wrong size of parentheses


\end{document}