My Question or Issue
The 1809 release of Windows 10 included the ability to make your OneDrive folders the default location for Documents, Music, Downloads, etc. After enabling this it appears that Spotify was not able to find any of the music in these folders. I also was not able to specifically add the OneDrive folder, Spotify just shows "Music Library" with a toggle. When I copied the files to my actual local Music folder, they showed up. I believe this is a bug and the software needs to be updated to understand this change in Windows 10.
Solved! Go to Solution.
Turns out the issue was simply that the files hadn't been downloaded in the cloud cache (the new handy cleanup for Windows 10 OneDrive cache deleted them because I hadn't listend to them for a while), and so copying them locally downloaded them also to the OneDrive folder and they showed up twice in Spotify. Gotta love Microsoft sometimes...