Having just updated the man pages I thought this was something I had done, but no - and it was building fine recently when I was testing this, downgrading asciidoc to 9.1.1-4 sees it build again.
Adding -v to the a2x command options in pakku/Makefile gives a little more output, but not sure of the fix yet...
${MAN_PAGES:=.in}: ${MAN_PAGES:=.txt}
@echo "GEN: $@"
@a2x \
-v \
--doctype manpage \