pause with transparent rounded shadow blocks rendered incorrectly

This simple LaTeX code is not rendered as one may expect.

documentclass{beamer}
setbeamercovered{transparent}
setbeamertemplate{blocks}[rounded][shadow=true]
begin{document}
begin{frame}
  begin{block}{A first block}
    Something inside    
  end{block}
  pause
  begin{definition}
    A nice definition
  end{definition}
end{frame}
end{document}

It gives something like that when compiled with pdflatex.

When the pause is removed it is render correctly as that.

The trouble also disappear if I remove the rounded aspect of blocks.

Is this a bug or did I miss something in the Beamer documentation?