I can always sync playlists fine from scratch, yet after disconnecting/reconnecting it trys to resync everything that's already on the phone showing the files 'In Queue' and the files already on the phone as 'Delete on Sync'. If I go ahead and action this sync, it freezes/MC25 crashes and appears to lose all the music on the phone... on reconnecting it's all there, along with newly created duplicate .DUP0 files for music and playlist files, taking up double the space. The only way I can reliably sync everytime is to delete all music and playlists from my phone's SD card and then sync from scratch. All I can think of is that the S7 is somehow modifying the files, so when reconnecting to MC it wants to add the files again.
I have now started using an SD card reader which works well, it only means you have to remove the SD card from the phone each time you'd like to sync. The sync also isn't as 'intelligent' as with a Fiio portable device, for example when changing a track to include Track# in the filename, it doesn't get picked up on in the next sync.