Spotify was syncing about 90% of my songs from my Local Files to my phone. When I looked at the problem the songs that weren't syncing behaved like a normal file rather than a Local File. They had no local file symbol in playlists, the context menu didn't have options such as open file location. So I get if Spotify has a better version of my local file then use that, fine. and hey disabling local sync shouldn't matter since I can just stream the song. BUT if those songs aren't available in my region and you disable local sync and block streaming on devises without the mp3 then you might have a few more angry customers.
It seems like a simple logic bug that shouldn't be that hard to fix any number of ways.