after a penalty the real space code falls back to the default font

\DocumentMetadata{uncompress,xmp=false,testphase=phase-III}
\documentclass{article}
\usepackage{fontspec}
\setmainfont{texgyreheros}
\begin{document}

a\ b

a\penalty10\ b

\end{document}

The first give three chars

[<001D00680024>]

but the second contains a font switch:

/F27 9.96264 Tf
1 0 0 1 148.712 645.28 Tm [<001D>]TJ
/F15 9.96264 Tf
1 0 0 1 154.252 645.28 Tm [<0067>]TJ
/F27 9.96264 Tf
1 0 0 1 157.021 645.28 Tm [<0024>]TJ
ET

latex3 / tagpdf

after a penalty the real space code falls back to the default font #72