The rc script sets the STNORESTART environment variable, which should only be set if the init system restarts the daemon automatically. So when you click the button to restart in the web UI, syncthing just exits.
Auto-assigned to maintainer swills@FreeBSD.org
A commit references this bug: Author: swills Date: Sat Oct 18 14:08:53 UTC 2014 New revision: 371125 URL: https://svnweb.freebsd.org/changeset/ports/371125 Log: net/syncthing: remove STNORESTART from rc script PR: 194442 Reported by: Andy Balholm <andy@balholm.com> Changes: head/net/syncthing/Makefile head/net/syncthing/files/syncthing.in
This was needed with older versions but seems not to be needed any more. Fix committed, thanks!