texorpdfstring with utf character causes "Label(s) may have changed"

[teepeemm]

\documentclass{article}
\usepackage{unicode-math}
\usepackage{hyperref}
\begin{document}
%\section{Solving µy=0} % ok
%\section{Solving λy=0} % not ok; also "Missing character" warning
\section{Solving \texorpdfstring{$l y=0$}{λy=0}}
\label{seclabel}
content
\end{document}

has a persistent "LaTeX Warning: Label(s) may have changed. Rerun to get cross-references right." If I use https://tex.stackexchange.com/a/169245/107497, it shows that the label is macro:->{1}{1}{Solving \texorpdfstring {$l y=0$}{λy=0}}{section.1}{} before and after. If I take out the unicode-math, things are fine. If I take out the texorpdfstring and just have \section{λy=0}, the persistence stops, although I also get a "Missing character" warning and the λ isn't in the output, so I'm not sure if that's related.

[u-fischer]

It is not \texorpdfstring or hyperref (or only because it loads nameref and so the title is stored in the label).

The problem is unicode-math: it changes lots of char in the begindocument hook after the aux-file has been read in. Beside others it changes the catcode of λ and so at the end of the document, when LaTeX compares the aux-files, they are different.

Work-arounds: change the catcode in the preamble or use lua-unicode-math.

\documentclass{article}
\usepackage{unicode-math}
%\catcode`λ=12
\begin{document}
\makeatletter
\section{Solving}\def\@currentlabelname{Solving λ}
\label{seclabel}
content
\end{document}