`\xrightarrow` with mathml-SE produces warnings

[mbertucci47]

One of the issues I was able to isolate from #918.

\DocumentMetadata
  {
    lang=en-US,
    pdfversion=2.0,
    pdfstandard=ua-2,
    tagging=on,
    tagging-setup={math/setup=mathml-SE},
  }
\documentclass{article}
\usepackage{unicode-math}

\begin{document}
$\xrightarrow{f}$ % same with \xleftarrow
\end{document}

warns

Package tagpdf Warning: Parent-Child 'mathml:mo' --> 'mathml:math'.
(tagpdf)                Relation is not allowed! on line 7
(tagpdf)                struct 32, mo --> struct 33, math


Package tagpdf Warning: structure with label 8 has already been used