The hook installed by avahi-autoipd unconditionally runs avahi-autoipd -k when dhclient assigns an IP. When the interface was not configured beforehand, no daemon is running, and the -k invocation will cause an exit code of 1. This in turn causes ifup to exit with 1, and the interface will end up in a half-configured state, and the ifup@.service service will show an error state.
-ch
