The problem is that the init script uses the deprecated runscript shebang, should be converted to openrc-run.
This was already fixed with https://gitweb.gentoo.org/repo/gentoo.git/commit/net-dns/ddclient/files?id=761bdd5b33e542ab3127f649fd3caa2cca0672b2 in May 2016. You may need to rebuild ddclient/upgrade to get the updated file.