Similar what happened to me years ago - i got new iphone, installed itunes, it synced all local files. Few weeks later, i de-synced the phone or something like that, and all my local files were gone! Still not sure how or why, there were no warnings or anything. From that day, i stay as far as possible from iTunes and similar "smart music sync" apps. Spotify doesn't see my local music collection, and vice versa. All music streaming services and their apps are total crap, they try to lock you in and manage everything like you're an idiot.