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

\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.

Start by compiling the minimal example in the issue, then compare runs with and without unicode-math and texorpdfstring. Trace why the label differs between runs; done means the example no longer produces a persistent rerun warning while retaining the intended λ PDF bookmark text.