Problem with shadow in Beamer's blocks, via dvips and ps2pdf

When I compile that code:

documentclass{beamer}
usetheme[]{Madrid}
useinnertheme[shadow]{rounded}
setbeamertemplate{navigation symbols}{}
begin{document}
begin{frame}[plain]
frametitle{Question 1 hspace{3cm} Développement}
begin{exampleblock}{Forme développée de }
begin{center}
bigskip
huge
$(4x-8)(2x+3)$
bigskip
end{center}
end{exampleblock}
end{frame}
end{document}

1/ with pdflatex, no problem

2/ with latex + dvips + ps2pdf, the shadow is incorrect (see the picture)

It seems the problem is in GPL GhostScript. With GhostScript 9.21, I have no problem.
In TexLive, it’s GhostScript 9.26 and, yes, there is a problem…

What can I do ?

Best regards

François