The lang/erlang-man port installs in /usr/local/etc/man.d the erlang.conf which contains "MANPATH /usr/local/lib/erlang/man". The path in this file is incorrect, it should had been "MANPATH /usr/local/lib/erlang".
The manifest shows that either the erlang.conf file is incorrect, or that the man categories directories aren't installed in the correct location as shown by the pkg manifest:
$ pkg info -l erlang-man-19.3 | head
Fixed back in 514ee195d6e2b590cc1e29b1e7cc1076e995c760.