When building graphviz without allowing fallback to FreeBSD's distfile cache the build fails during the fetch stage  (i.e. upstream is no longer serving up the source files).
It seems the source location has been changed to https://graphviz.gitlab.io/pub/graphviz/stable/SOURCES/graphviz.tar.gz
A commit references this bug:
Date: Sun Sep 16 05:48:53 UTC 2018
New revision: 479876
- update MASTER_SITES