By accident, I found out, that apparently executing "rpm --eval %_mandir" reports wrongly "/usr//usr/share/man". The background was, that dev-util/statifier was using as fallback "rpm --eval %_mandir" to get the man page dir, which is anyway wrong. Could be related to wrong EPREFIX?
This seems present in all versions in tree: 4.20.0: $ rpm --eval %_mandir /usr//usr/share/man 4.20.1: $ rpm --eval %_mandir /usr//usr/share/man