pgfopts .unknown does not handle key-value pairs?

Question

I would like my custom class to forward unknown keys to the base class, or in this case, a package.

The following works (to make the example smaller, this forwards only one key; expanding it to a list would clutter the code):

\NeedsTeXFormat{LaTeX2e}
\ProvidesClass{mwe}[2020/02/17 MWE]

\LoadClass{beamer}

\RequirePackage{pgfopts}
\def\@unknownkeys{}

\pgfkeys{
    /mwe/.cd,
    .unknown/.code={
        \message{mwe found unknown key: \pgfkeyscurrentname}
        \edef\@unknownkeys{\pgfkeyscurrentname}
    }
}

\ProcessPgfPackageOptions{/mwe}
\relax

\PassOptionsToPackage{\@unknownkeys}{geometry}
\RequirePackage{geometry}

Now the following document:

\documentclass[showframe]{mwe}
\begin{document}
foo
\end{document}

displays a frame, as expected.

But if I modify my code by replacing

\edef\@unknownkeys{\pgfkeyscurrentname}

by

\edef\@unknownkeys{\pgfkeyscurrentname=\pgfkeyscurrentvalue}

then it does not work anymore:

\documentclass[paper=a0paper]{mwe}
\begin{document}
foo
\end{document}

gives me a DIN-A4 document. What's more, the custom message does not appear in the log, showing that the .unknown/.code was never executed at all.

Why is this? Am I doing something wrong?

PS: I originally had code similar to that provided in this answer, but that does not work, as you can see above.

Edit: forgot a line in my MWE.

Answer 1

I figured out that it is the class beamer which kills it. Moving the \LoadClass statement to after the option processing makes it work as expected.

My best guess is that beamer uses some .unknown handler internally itself. I would still be glad for a more qualified explanation though.