Scaling inside a TikZ matrix does not work?

This is one of those grey areas about cell picture issues. One not-super-clean solution is to set the transformation yourself.

\documentclass[margin=5pt,tikz]{standalone}
\begin{document}

    \begin{tikzpicture}[scale=.5]
    \pgfgettransform\mytrafo
    \matrix [yshift=2cm,execute at begin cell=\pgfsettransform\mytrafo]{
       \draw (0,0)--(0.2cm,1cm)--(1cm,1.2cm)--(1.2cm,0)--cycle;&
       \node{Some text that should not be scaled};\\
    };

    \draw (0,0)--(0.2,1)--(1,1.2)--(1.2,0)--(0,0);
    \end{tikzpicture}

 \end{document}