Potential regression in natbib/beamer citation formatting in TeXLive 2026

Dear TeXLive Team,

I am writing to report a potential bug regarding citation formatting that
appears when using the TeX Live 2026 distribution (verified on
texlive.net/run).

*Description:* When using natbib with the beamer document class, the
citation punctuation (specifically the comma and parentheses placement) is
not being rendered correctly.

   -

   *Observed output:* (Vaswani et al.2017) and Vaswani et al.2017
   -

   *Expected output:* (Vaswani et al., 2017) and Vaswani et al. (2017)

It seems the round and authoryear options in natbib are being ignored or
overridden in this environment. Downgrading back to TeXLive 2025 solves the
issue.

*Minimal Working Example (MWE):*
\begin{filecontents}{refs.bib}
@article{vaswani2017attention,
  title={Attention is all you need},
  author={Vaswani, Ashish and Shazeer, Noam and Parmar, Niki and Uszkoreit,
Jakob and Jones, Llion and Gomez, Aidan N and Kaiser, {\L}ukasz and
Polosukhin, Illia},
  journal={Advances in neural information processing systems},
  volume={30},
  year={2017}
}
\end{filecontents}

\documentclass{beamer}
\usepackage[authoryear,round]{natbib}

\begin{document}

\begin{frame}{Introduction}
\begin{itemize}
    \item Observed \citep{vaswani2017attention} (Missing comma)
    \item Observed \citet{vaswani2017attention} (Missing parentheses around
year)
\end{itemize}
\end{frame}

\begin{frame}[fragile]{References}
\bibliographystyle{plainnat}
\bibliography{refs.bib}
\end{frame}

\end{document}

*Environment Info:*

   -

   *Distribution:* TeX Live 2026 (released March 2026)
   -

   *Compiler:* pdfLaTeX
   -

   *Platform:* Verified via https://texlive.net/run, MacOS

Best regards,

Steven