The default character set of xmlto for man pages seems to be locale-dependent. The patchutils-0.2.30 tarball contains man pages in iso-8859-1. It might be better to run xmlto as (LC_ALL=en_US.UTF-8 xmlto ...) * to make builds more reproducible * because UTF-8 is The One Right Encoding (-: and man complains when man pages are not in UTF-8.
The problem was in docbook-style-xsl. Fixed in docbook-style-xsl-1.67.0-3 and patchutils-0.2.30-3.