Using \EveryShipout corrupts the copyable text

Question

This is a strange bug. I'm using the textpos package (which in turn uses the everyshi package) to place text at a certain position on the page. Consider this document:

% !TEX TS-program = lualatex
\documentclass{article}
\usepackage[absolute]{textpos}
\usepackage{fontspec}
\setmainfont[Renderer=HarfBuzz,Script=Devanagari]{Chandas}
\begin{document}
% package textpos uses \paperheight and \paperwidth
\paperheight=1561bp
\paperwidth=1044bp
\TPGrid{1044}{1561}
\begin{textblock}{200}(173,213)
added later किं  बहुना । परस्परं द्वैधम् उत्पन्नम् । added later
\end{textblock}
Some text.
\newbox\foo
\setbox\foo=\vbox{\hbox{boxed किं  बहुना । परस्परं द्वैधम् उत्पन्नम् । boxed}}
inline किं  बहुना । परस्परं द्वैधम् उत्पन्नम् । and also below
\unvbox\foo
\end{document}

(The Chandas font is from here.) The resulting PDF looks like:

The Devanagari text is the same in all three places. As in my previous question Getting correct searchable text for Devanagari text, everything works fine (when compiled with lualatex from Tex Live 2020: see on Overleaf) i.e. selecting and copying text from the PDF gives correct results, for the inline text and for the manually unvboxed text,

inline किं बहुना । परस्परं द्वैधम् उत्पन्नम्। and also below

boxed किं बहुना । परस्परं द्वैधम् उत्पन्नम्। boxed

but for the text added later with textpos, I get:

added later ��� बहु�� । परस्पर� ��ध�� उत्पन्न�� । added later

What could be the reason? Is there a fix?