I just reinstalled a new SD card on my 2019 Samsung Galaxy Tab A and tried to sync from my MMW ( I also just updated from MMW4 to MMW5). The tablet shows the Megadata syncing and then says sync complete. None of the music files transferred however. I had kind of the same issue before. Going from MMW...

I have kind of the same problem. MM picked up some of the songs from the folders but not all. Repeated attempts at "Add/rescan files..." does not correct the problem. My music is all stored in my main directory on my laptop and I've only noticed it on a couple of folders.