I’m wondering if this is related to an issue I raised with the author of MPD a while ago:
So what happens is, when Volumio sends the path of the DSF file, which happens to be a network path, to MPD for playback, the metadata is not read for reasons indicated in the Github issue. This explains why you get the correct title, artist, etc. when playing the same file locally on Volumio but not from the NAS.
If Volumio has the information about the DSD, it can actually show the correct title, artist, etc. by sending the relevant tags to MPD. It’s up to the developers to decide whether they want to do this.
Personally, I won’t rely on Volumio to manage my media. I have been using Jellyin media server instead and wrote a plugin to play songs hosted on the server. So far so good and yes - titles, artists and album art are displayed correctly for DSFs.
I invite you to try it and you may even be able to install Jellyfin on your Synology NAS (haven’t tested this myself - I’m running mine on a NanoPi with Armbian).