% Public domain. From Ulrike:
% https://tug.org/pipermail/pdftex/2024-July/009424.html
% Original problem, that spaces are lost at font switches.

\pdfcompresslevel0
\pdfobjcompresslevel0
\font\test=cmss10
\pdfinterwordspaceon

text text {\test cmss cmss} text text

\end

% there is a space char between "text text" and "cmss cmss":
%
%    [(text)]TJ/F51 9.9626 Tf( )Tj/F1 9.9626 Tf 20.756 0 Td [(text)]
%    [(cmss)]TJ/F51 9.9626 Tf( )Tj/F20 9.9626 Tf 23.302 0 Td [(cmss)]
%
% But nothing between "text cmss" and "cmss text"
%
%    [(text)]TJ/F20 9.9626 Tf 20.755 0 Td [(cmss)]
%    [(cmss)]TJ/F1 9.9626 Tf 23.301 0 Td [(text)]