% Public domain. From Ulrike Fischer:
% https://tug.org/pipermail/tex-live/2025-February/051136.html

\ExplSyntaxOn\sys_ensure_backend:\pdf_uncompress:\ExplSyntaxOff

\documentclass{article}

\begin{document}
\pdfinterwordspaceon
$ a b \mathbf{u} $ $ a\mathbf{u} $ $ a\mathbf{b} $

\end{document}

% When I compile this the first equation is correct. But the second is
% missing the bold u, and the third uses an mathitalic b instead of a
% bold b.