\tcblisting uses alphanumerical foonote instead of numerical?

Question

Why is \tbclisting using alphanumerical notation of footnotes instead of numerical?

\documentclass{article}
\parskip\baselineskip
\usepackage{tcolorbox}
\tcbuselibrary{listings}
\begin{document}
\begin{tcblisting}{before lower={\vspace{-1em}\parskip\baselineskip}}
This is a \footnote{footnote}
\end{tcblisting}
This is a \footnote{footnote}
\end{document}