Hi, consider this
\documentclass{article} \usepackage{xstring} \begin{document} \end{document} and call it xexstring.tex Then xelatex xexstring triggers 136 warnings of the type Invalid UTF-8 byte or sequence at line 35 replaced by U+FFFD. Looking at file /usr/local/texlive/2020/texmf-dist/tex/generic/xstring/xstring.tex I see that this matches with use of latin-1 encoded characters in comments. Notice that it is a not a user decision here to use a latin-1 encoded file. In fact I encountered this in a file I was given where xstring package was loaded by another package. Regards, Jean-François