When you connected to a library you are changing the frame of reference to the tree so we thought this approach made more sense.
I not only "just" listen to music, I sometimes sit in front of the computer and the music managing software (which is now MC) and browse the albums and artists. I rip CDs or buy FLACS on the internet, tag them properly (very properly), scan in a cover. I often listen to new music which I propably will delete right after having listened to it, rate it, re-rate it, change some other file tags, listen to a special song over and over again, etc. So I do use the software. After they were able to sort albums by year, I really loved iTunes and the way it managed music files and playlists. But now Apple is more in selling stuff and iTunes is too much for what I like, as you can't ignore the parts of the software you won't need any more. And I switched to FLAC which is not supported (any more - there was a plug-in). MC, in the GUI, is not as comfortable as iTunes but has anything I need and much more than iTunes will ever offer and I can forget the video part, when I don't need it.
Well, what I'm trying to say is, that now it's complicated to access the media on the NUC, as it needs more clicks to play files and it's not quite clear why it is that way. I only do have one library, I play music from and that's the one on the NUC now. To me there's no need for an extra way.
With your flac file, I wonder if the & is the issue. It's a special character in linux. Are all of the files that won't update like that (or with other characters like backquotes, etc)?
I had no problems with all my files on my Synology NAS before with the "&" symbol. I know there are certain chracters that can make problems, but those I already avoid. But I can't check that any more. Now that I know how to remote the library on the NUC, I started it in headless mode and put it underneath the couch.
And: I'm quite happy with MC, although there is still place for improvements.