I am off to bed in a minute, so I won't reply again tonight, I get the impression that Xbox music needs to be approached with a bit of forethought especially if you have a large local collection, I have always copied my tracks/albums from one device to another and stayed clear of letting the system sync my files.
The one setting I don't have on is "Automatically Match my songs to the cloud".
If this is or has been on, it may pay you start again if that is what is causing issues.
Another thing to consider is having your "Music" folder as the only folder Xbox Music looks for music, we had an issue when I set my wife up with a music pass because the system was trying to put music onto a shared network drive.
I have to say, I have had a very good experience with Xbox music on my PC's, phone and now my Xbox One, but it did take a bit of thinking through before I committed to it, and as mentioned I then took out a music pass, tripled my collection in a few months and just bought a 128G SD card to replace my fast filling up 32G and the whole lot works pretty well together.
You will find lots of people who moan about Xbox music, I think a lot of this is down to how they go about the initial setup.
Good luck and I will be about tomorrow if I can help although I am just a casual user and no expert.