Footnote modification (or hack, really) leads to tagging error

[John02139]

Perhaps I simply shouldn't hack things this way, but in case it's useful to know ...

Changing \@makefntext as shown has worked fine for years. With phase-III, however, I get an undefined control sequence error -- which I can eliminate by setting the hack to \AtBeginDocument{..}.

\DocumentMetadata{ 
    testphase = {phase-III},
}
\documentclass[10pt]{article} 

\makeatletter
%% reduce indentation of footnotes
%\AtBeginDocument{
\RenewDocumentCommand\@makefntext{m}{%
  \noindent\makebox[2ex][r]{\@makefnmark}#1}
%}
\makeatother

\begin{document}
\section{Preliminary remarks}
Introductory comments with a humorous anecdote.

And here's what it really means.\footnote{A footnote}
\end{document}

% If \AtBeginDocument{..} is NOT used, the following error appears with tagging:
%
%! Undefined control sequence.
%<argument> \ERROR 
%                  \cs_set_eq:NN \__fnote_tmp:w \exp_stop_f: 

Uncommenting \AtBeginDocument{..} clears the error.

[u-fischer]

You will break the tagging if you redefine \@makefntext like this. It is not very obvious but if you look carefully you can see that the label (the 1) is not in the footnotelabel structure but in the text itself:

You can probably get your wanted output simply by setting \setlength\footnotemargin{2ex}, if that is not enough you must define a new plug for the fntext/mark socket.

[John02139]

The simple (and in hindsight, very obvious) fix of \setlength{..} resolves the problem for me.

[John02139]

Perhaps worth mentioning that \footnotemargin is defined in testphase={phase-III} but otherwise requires loading the package footmisc.