To reproduce, download the attached pdf file, formatted from the LaTeX code below (compiled with luaLaTeX):
```
\DocumentMetadata{
lang = en,
pdfstandard = ua-2,
pdfstandard = a-4f, %or a-4
tagging=on,
tagging-setup={math/setup=mathml-SE}
}
\documentclass{article}

\begin{document}
This produces an error with screen readers:

$x < 2$
\end{document}

Open it in Adobe Acrobat (I'm using Acrobat standard 2025.001.20844, Windows 64-bit), then attempt to read with a mathML-capable screen reader (I'm using NVDA 2025.3.2+mathCAT 0.7.5).
It gives an illegal mathML error. Looking in the NVDA log:

OSError: Invalid MathML input:
<math id="ID.010"><mi id="ID.011">x</mi><mo id="ID.012"><</mo><mn id="ID.013">2</mn></math>
Error is: XML parsing error at 57: {ExpectedElementName}
```
Notice that the '<' character in the pdf structural elements was not escaped as <, resulting in illegal xml. I assume this is an error on Adobe's side, as I encountered the same error with Access8math (another NVDA add-on for mathML).