beamer and tikz, use of \uncover

[Zarko F. Cucej]

Hi,
consider minimal example below. why part of image on background layer is
not effected with \uncover command? do I miss something or this is bug?

regards, Zarko

\documentclass{beamer}
\usepackage{tikz}
\usetikzlibrary{backgrounds,fit,shapes}
\pgfdeclarelayer{background}
\pgfsetlayers{background,main}

\begin{document}

\begin{frame}
\begin{tikzpicture}
\uncover<1->{
\node[rectangle,draw,draw,fill=gray!50] (A) at (0,0) {abc};
\begin{pgfonlayer}{background}
\node[inner sep=3mm,draw,fit=(A)] {};
\end{pgfonlayer}
}
\uncover<2->{
\node[rectangle,draw,draw,fill=gray!50,below=11mm] (B) at (A) {def};
\begin{pgfonlayer}{background}
\node[inner sep=3mm,draw,fit=(B)] {};
\end{pgfonlayer}
}
\end{tikzpicture}
\end{frame}

\end{document}