shadow.service was reporting failure, so I ran grpck & pwck, of which pwck reported that /var/lib/mpd did not exist. sudo mkdir /var/lib/mpd was ran, and now shadow.service consistently reports success.

I assume this is due to the fact that I exclusively run a non-root instance of mpd, where my own config is being used and the system one (which tells mpd to store info in /var/lib/mpd, which I assume mpd creates if it doesn't exist) is being ignored.