optional argument in align breaks tagging structure

[u-fischer]

\DocumentMetadata{
 lang=en,
 testphase={phase-III,math,table,title}}

\documentclass{article}
\begin{document}
\begin{align*}
x& \begin{tabular}{c}
 $a$ 
\end{tabular} 
\\[2ex]
\end{align*}
\end{document}

gives a warning

Package tagpdf Warning: There are still open structures on the stack!
(tagpdf)                The stack contains
(tagpdf)                {Document}{Document},{Root}{StructTreeRoot}.
(tagpdf)                The structures are automatically closed,
(tagpdf)                but their nesting can be wrong.

The optional argument [2ex] triggers a subpart:

====>first-result=macro:->x& \begin {tabular}{c} $a$ \end {tabular} 
====>first-tmpmathcontent=macro:->[2ex]
====>formula has subparts
====>first-result=macro:->a
====>first-tmpmathcontent=macro:->
====>formula has no subparts

Without the optional argument the structure is correct. It is also correct if the inner tabular doesn't contain math.

[u-fischer]

The error mainly affects pdflatex. And there are imho two general questions here:

• should subpart processing (the splitting of formulas at the \\) be deactivated by default? We are not using it anymore if a mathml file exists or luamml is used, and the results are a bit dubious.
• should there be further math grabbing inside the align? (the $a$ inside the tabular is marked as a Formula).

@FrankMittelbach any thoughts?