error if math structure elements are created in fake math

[u-fischer]

fake math like an url is not tagged as Formula but luamml tries to create math structure elements and fails:

\DocumentMetadata{testphase={phase-III,math},pdfversion=2.0}
\documentclass{book} 
\usepackage{url}
\tagpdfsetup{math/mathml/structelem}
\tagpdfsetup{math/mathml/luamml/load}
\begin{document}

\url{https://www.latex-project.org}

$a=b$

\end{document}

Package tagpdf Warning: Parent-Child 'P/pdf2' --> 'math/mathml'.
(tagpdf)                Relation is not allowed (struct 6, /text --> struct 7)
(tagpdf)                on line 3


! Package tagpdf Error: attribute /luamml_attr_1 is unknown