[XeTeX] "Missing character" warnings in beamer

[Michael Ring]

Dear all,

when I run the following sample in XeLaTeX (TeX Live 2021)

\documentclass{beamer}
\usetheme{Frankfurt}
\begin{document}
\begin{frame}{Test}
\begin{block}{test block}
\end{block}
\end{frame}
\end{document}

it outputs the following warning messages:

Missing character: There is no 4 ("34) in font nullfont!
Missing character: There is no 1 ("31) in font nullfont!
Missing character: There is no 0 ("30) in font nullfont!
Missing character: There is no 4 ("34) in font nullfont!
Missing character: There is no 4 ("34) in font nullfont!

The output is not broken at all, I just find these warnings annoying.
They do not appear with pdfLaTeX or LuaLaTeX, so I thought it was a
XeTeX-related issue.
I have already tried to replace

\usetheme{Frankfurt}

by the content of the Frankfurt theme file. There are the two lines

\useoutertheme[subsection=false]{smoothbars}
\useinnertheme[shadow=true]{rounded}

If I comment both lines out, the warnings will all disappear, but then,
the output is no more what I want. I've also found out that the number
of warnings decreases by
- commenting one of these two lines out,
- removing the block, or
- setting [shadow=false] in the \useinnertheme line

But as I am not familiar with internals, I didn't dig deeper.

Is this a known bug and where does it come from? XeTeX? Beamer? PGF?

Thanks for your help,
Michael