The mk.conf file wasn't being created if only mkconf_lines was defined. The check was only being made for mkconf_tail.
Tim. Index: proot =================================================================== RCS file: /cvs/ports/infrastructure/bin/proot,v retrieving revision 1.54 diff -u -p -r1.54 proot --- proot 26 Jun 2016 22:10:03 -0000 1.54 +++ proot 22 Sep 2016 01:09:54 -0000 @@ -1110,7 +1110,8 @@ if ($state->{actions}{stragglers}) { $state->verbose_shit; } if ($state->{actions}{write_mk} && - exists $state->{write} || exists $state->{mkconf_tail}) { + exists $state->{write} || exists $state->{mkconf_tail} || + exists $state->{mkconf_lines}) { $state->write_mk_conf; } if ($state->{actions}{check_symlinks}) {