When I align two minipages together, the contents location changes after a figure in one minipage
is uncovered. The distance from the rule to the upper palettes illustrates this
\documentclass[aspectratio=169, xcolor={x11names}]{beamer}
\setbeamercovered{transparent=30}
\usecolortheme{seahorse}
\useoutertheme{miniframes}
\useinnertheme{inmargin}
\usepackage{adjustbox}
\usepackage[defaultmathsizes, subdued, italic, symbolre, symbolmisc]{mathastext}
% BEGIN_FOLD
\MTDeclareVersion[it]{charter}{T1}{bch}{m}{n}
\MTversion*{charter}
% END_FOLD
\begin{document}
\begin{frame}
\uncover<+->{$\vphantom{1}$}
\rule{\textwidth}{0.2mm}
\begin{adjustbox}{minipage={0.45\textwidth}, valign=t}
\includegraphics<+->[trim = 0mm 0mm 0mm 0mm, width=\columnwidth]{example-image-a}
\end{adjustbox}
\hfill
\begin{adjustbox}{minipage={0.45\textwidth}, valign=t}
\begin{itemize}
\item<+-> Modeling the addition of branch $Z_{b}$ between two existing buses \mbox{($k$ and $p$)} in Brown's method
\end{itemize}
\end{adjustbox}
\end{frame}
\end{document}