Missing space key in frametitle of beamer

Question

Why is there a space ' ' missing in the title, although I wrote \frametitle{\insertsection - \insertsubsection} with the equivalent space.

Here is my MWE:

\documentclass{beamer}
\usetheme{Boadilla}
\begin{document}
\section{Section}

\subsection{Subsection}

\begin{frame}
    \frametitle{\insertsection - \insertsubsection}
    Frame
\end{frame}


\end{document}