Extracting height of minipage with subfloats inside

Question

I am repeating an example shown previously in One figure having two columns of subfigures: the left column has one image and the right one has two images , but with the use of subcaption package.

Finally, I got one failed version and one successful version. My question is, why do I fail for the first version (getting the wrong height of minipage with subfigure environment inside)?

% The failed version
\documentclass{article}
\usepackage{subcaption}
\usepackage[demo]{graphicx}
\newsavebox{\measurebox}
\begin{document}
    \begin{figure}
        \centering
        \sbox{\measurebox}{%
            \begin{minipage}[b]{.33\textwidth}
                \begin{subfigure}{\textwidth}
                    {\includegraphics[width=\textwidth,height=5cm]{figA}\caption{caption A}}
                \end{subfigure}
              \end{minipage}}
        \fbox{\usebox{\measurebox}}\qquad
        \fbox{%
            \begin{minipage}[b][\ht\measurebox][s]{.33\textwidth}
                \centering
                \begin{subfigure}{\textwidth}
                    \includegraphics[width=\textwidth,height=2cm]{figB}\caption{caption B}
                \end{subfigure}
                \vfill
                \begin{subfigure}{\textwidth}
                    \includegraphics[width=\textwidth,height=2cm]{figC}\caption{caption C}
                \end{subfigure}
            \end{minipage}%
        }
        \caption{my caption}
    \end{figure}
\end{document}

% The successful version
\documentclass{article}
\usepackage{subcaption}
\usepackage[demo]{graphicx}
\newsavebox{\measurebox}
\begin{document}
    \begin{figure}
        \centering
        \sbox{\measurebox}{%
            \begin{minipage}[b]{.33\textwidth}
                \includegraphics[width=\textwidth,height=5cm]{figA}\subcaption{caption A}
            \end{minipage}}
        \fbox{\usebox{\measurebox}}\qquad
        \fbox{%
            \begin{minipage}[b][\ht\measurebox][s]{.33\textwidth}
                \centering
                \includegraphics[width=\textwidth,height=2cm]{figB}\subcaption{caption B}
                \vfill
                \includegraphics[width=\textwidth,height=2cm]{figC}\subcaption{caption C}
            \end{minipage}%
        }
        \caption{my caption}
    \end{figure}
\end{document}