in minipage if center vertical alignment is used then \vcenter is used and so math is used. Since thing are boxed in advance it is mostly unnoticeable, as the direction registers are correct at the time of the boxing, but the direction of the \vcenter arguably wrong, and the alignment of boxes inside are according to \mathdirection:
\documentclass{article}
\usepackage[hebrew,bidi=default]{babel}
\parindent=0pt
\begin{document}
\begin{minipage}{\textwidth}
foo
\hbox{foo}
\end{minipage}
\begin{minipage}[t]{\textwidth}
foo
\hbox{foo}
\end{minipage}
\end{document}
in minipage if center vertical alignment is used then
\vcenteris used and so math is used. Since thing are boxed in advance it is mostly unnoticeable, as the direction registers are correct at the time of the boxing, but the direction of the\vcenterarguably wrong, and the alignment of boxes inside are according to\mathdirection: