Please press the edit button to bring you in preview/edit mode. See what happens in the preview area. The code markdown does not work well in case A, i.e., the first code line is not displayed as code but as normal text.
In case B, I insert any object between the heading and code markdowns, now everything works as expected.
I am using Google Chrome and Firefox on Windows, I confirm it might be a bug.
Case A: Heading markdown is immediately followed by a code markdown
\documentclass{minimal}
\usepackage{pstricks}
\begin{document}
.%my code goes here
\end{document}
Case B: Heading markdown is not immediately followed by a code markdown
=== any object can be here to make the code markdown work ===
\documentclass{minimal}
\usepackage{pstricks}
\begin{document}
.%my code goes here
\end{document}