I have a Note 10 + and I had the Wifi sync app to easily sync files onto my phone. Note that I put all my music files on my SD card so that when changing phones all I need to do is move the SD card.
A few days ago the Wifi sync stopped working correctly, it would start the sync then say it was completed but no new files were getting transferred. I even deleted a playlist and started the sync. The playlist showed up but with no tracks in it. I then tried syncing it a few more times and it still would not sync the tracks. I then tried a hardwired connection. For some reason it kept telling me my phone was unresponsive and it was unable to sync any tracks. Then I noticed that my phone was listed twice in the "devices" tab so I re-established all the syncing options and had to get some sleep.
Today I went to sync it and it sort of worked, except this time it copied every song over again to the SD card so every track was duplicated and it filled up my SD card. The only way to correct the issue was to completely delete all the music files on my phone and re-sync everything, so far it seems to be working and is currently at 88%.
It finished syncing but now I am having an issue where it isn't copying the playlists correctly. Every playlist shows up but they are listed with "0" (zero) tracks. I deleted all the playlists (but not the actual music tracks) on the SD card am currently trying to re-sync but it seems to be taking much longer than before.