latin1 files
The file xstring.tex used to be UTF-8 encoded (as distributed in texlive 2017) but current versions have comments in latin-1 (iso 8859-1) latex defaults to UTF-8 and in xelatex and lualatex there are only low level mechanisms not normally used by users to change the file encoding.
The non ascii characters are all in comments but still prevent the byte stream being properly decoded, and generate multiple warnings see