\begin{myenv} ended by \end{document}

Question

For a pandoc application, I want to use a custom environment. The environment's definition varies depending on a pandoc variable I defined (btw awesome feature!).

To manipulate the environment's content, I used the environ package as suggested here: Capture the content within an environment

To use markdown inside begin-end inside my source file, I used the trick suggested here: https://stackoverflow.com/a/41005796/1406122

MWE

\documentclass{article}

% some example environment
\usepackage{environ}
\NewEnviron{MyEnvironment}{%
%  Depends... :) %
%  \textsl{\BODY}
  This content is hidden.
}

% some example environment
\newenvironment{myenvironment}%
  {\bgroup\slshape}%
  {\egroup}

% hide stuff from PANDOC
\let\Begin\begin
\let\End\end

\begin{document}

Some text.

\Begin{MyEnvironment}
\begin{itemize}
\item Some itemized content
\end{itemize}
\End{MyEnvironment}

\end{document}

However, these two tips don't mix well!

LaTeX Error: \begin{MyEnvironment} on input line 24 ended by \end{document}.

If the standard environment myenvironment is used instead, the above example compiles as expected. Yet with normal environments I can't modify its body content.

P.S.: To give some context, this was translated from a source MD-file:

Some text.

\Begin{MyEnvironment}

- Some itemized content

\End{MyEnvironment}