Using iphone 6, everytime I update Spotify my songs I have syncd from my PC become unsyncd.  I did a Google search and this seems to have been an issue since 2013 which doesn't give me much hope that it'll be fixed any time soon.

I try to re-sync them, it does nothing

I tried turning "offline" on and off, it does nothing

I remove them from the playlist on my phone, and they won't sync back

I remove them from the folder on my PC, then added them back, it "sometimes" syncs back to my iphone

Right now I've removed the songs from my iphone playlist, removed the files from the folder, deleted the folder, removed the folder from local files in preferences and when I click local files, the songs are there with no way (that I can see) to remove them. Selecting them then edit > delete does nothing.

When I add the new folder back I see doubles of each of my local songs because I wasn't able to delete them in the first place.

Can someone explain to me how to add local music to a playlist in Desktop Spotify PC, then sync it to iPhone and not have the songs disappear with every update?