Current order of PACKAGESPLITFUNCS is:
populate_packages_updatercd populate_packages_updatealternatives systemd_populate_packages package_do_split_locales populate_packages

As You see populate_packages_updatercd is executed before populate_packages_updatealternatives.

As a result we might have following content of DEB package postinst script:

#!/bin/sh
if type update-rc.d >/dev/null 2>/dev/null; then
	if [ -n "$D" ]; then
		OPT="-r $D"
	else
		OPT="-s"
	fi
	update-rc.d $OPT syslog start 20 2 3 4 5 . stop 90 0 1 6 .
fi
	update-alternatives --install /etc/init.d/syslog syslog-init /etc/init.d/syslog.syslog-ng 200

update-rc.d will fail in case of absence of syslog target. But syslog target is created after update-rc.d execution.

As You see, update-alternatives should be called BEFORE update-rc.d call.

So at the end we should have this order of PACKAGESPLITFUNCS:
populate_packages_updatealternatives populate_packages_updatercd systemd_populate_packages package_do_split_locales populate_packages