pgf layers do not get cleared

[xworld21]

A baffling scope bug, seemingly related to some issues reported on ar5iv. I spotted this in the atoms-and-orbitals test: the content of pgf layers (created via pgfonlayer) does not clear when closing a group.

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
    \pgfdeclarelayer{background}
    \pgfsetlayers{background,main}
    \draw[red] (-1,0) -- (1,0);
    \begin{pgfonlayer}{background}
    \filldraw (0,0) circle (4pt);
    \end{pgfonlayer}
\end{tikzpicture}
\begin{tikzpicture}
    \pgfdeclarelayer{background}
    \pgfsetlayers{background,main}
    \draw[red] (-1,0) -- (1,0);
\end{tikzpicture}
\end{document}

renders as

You can see that the black circle is still in the background layer even if it's a new image.