The rc script installed by net/mpd5 (/usr/local/etc/rc.d/mpd5) fails to start the service in the scenario where the mpd_flags variable in /etc/rc.conf specifies an alternate location for the configuration directory or -file by using the -d/--directory or -f/--file options and the file /usr/local/etc/mpd5/mpd.conf does not exist. This is caused by the following line that disregards anything that might be specified in the mpd_flags variable: required_files="/usr/local/etc/${name}/mpd.conf"
A commit references this bug: Author: eugen Date: Sat Aug 29 20:15:41 UTC 2020 New revision: 546946 URL: https://svnweb.freebsd.org/changeset/ports/546946 Log: net/mpd5: remove required_files from startup script Name and/or location of the configuration file may be changed with mpd_flags, in which case required_files prevents starting service. It is no big deal if mpd5 started with missing configuration file because it just prints a warning to the log and exits in that case. PR: 248993 Changes: head/net/mpd5/Makefile head/net/mpd5/files/mpd5.in
Fixed with r546946. Thank you for the report!