With beamer, "Missing { inserted." (regression)

[Vincent Lefevre]

Under Debian/unstable with the TeX Live 2021 Debian packages
and with the old TeX Live 2020 Debian packages, pdflatex on

\documentclass{beamer}
\begin{document}
\begin{frame}
$e_\textnormal{m}$
\end{frame}
\end{document}

gives the following error:

! Missing { inserted.
<to be read again> 
                   \def 
l.5 \end{frame}

There are no issues with TeX Live 2018 under Debian 10 buster.
The author of the full .tex file uses a macOS machine, so I assume
that there are no issues there either (but I don't know which
TeX distribution it is).

-- 
Vincent Lefèvre <vincent at vinc17.net> - Web: <https://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)