frame in environment and note fail in beamer

Initialize \beamer@cramped in the preamble using

\makeatletter
\def\beamer@cramped{}
\makeatother

For your example, use it as

\documentclass{beamer}
\setbeameroption{show notes}

\newenvironment{myframe}%
{\begin{frame}[environment=myframe]\frametitle{SomeTitle} SomeContent}
{\end{frame}}
\makeatletter
\def\beamer@cramped{}
\makeatother

\begin{document}

%\begin{frame} Test \end{frame} % works
\begin{myframe} Test \end{myframe} % fails when compiling the note

\note[itemize]
{
    \item Note test
}

\end{document}