The netatalk3 port has a config switch to select either Avahi or mDNSResponder as the zeroconf implementation. However, the port's configure script automatically selects an implementation based on installed headers and libraries, and it prioritizes Avahi over mDNSResponder if both are installed.
In fact, it will pick Avahi even if only net/avahi-app, but not the actual avahi daemon from net/avahi is installed (which was the case on my machine) -- and in this case, the result is that registration with avahi fails and netatalk cannot advertise its services.
A commit references this bug: Author: marcus Date: Fri Aug 5 20:32:11 UTC 2016 New revision: 419708 URL: https://svnweb.freebsd.org/changeset/ports/419708 Log: Ensure that Avahi supprt is disabled if mDNSResponder is chosen for Zeroconf. PR: 211571 Changes: head/net/netatalk3/Makefile
The problem with mDNSResponder not being used should now be fixed. However, avahi-app does include avahi-daemon. So I don't think we need to switch the dependencies.