It would seem that the version of Mu4E available in the ports tree is now behind the version of Mu: 9.16 versus 9.18, respectively. Emacs throws an error message when trying to start Mu4E indicating this: "mu4e-error: [mu4e] mu server has version 0.9.18, but we need 0.9.16"
I sent an update to 1.0: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=226158
A commit references this bug: Author: hrs Date: Wed Feb 28 05:57:32 UTC 2018 New revision: 463178 URL: https://svnweb.freebsd.org/changeset/ports/463178 Log: Update to 0.9.18. PR: 225852 Changes: head/mail/mu4e/Makefile head/mail/mu4e/distinfo head/mail/mu4e/files/patch-configure.ac