Scroll down to next bold bit to skip rant and see tip that might help.
This has never worked well at any time, none of the touted improvements has made any difference, and it probably has no prospect of working because I've been saying this for years in the Google Play reviews and every single time they've just told me to update to the latest version which I obviously already had. And since I've always already spent at least six hours reinstalling and resyncing and have already told them the steps to reproduce ("sync to android") I generally am not in the mood by that point to continue the correspondence.
Yesterday - after replacing my SD card with a new one and syncing for two hours - it synced my music and ALL my playlists! I was overjoyed. I started to use them. They worked. And while I was using them - on a train - underground - in airplane mode - as I switched from playlist to Now Playing and back - ALL the playlists disappeared from the phone. Every single one. Playlists folder mysteriously disappeared from the list of folders, but even when brought back it showed as empty.
Second sync, and at least some of the playlists are there on the phone (nowhere near all the ones MM individually announced it was synchronising), but MM for android (reinstalled multiple times earlier in the day - this is my once-a-year weekend of trying to troubleshoot MM on the phone) doesn't see them. It sees some of them. It's not because there are playlists of playlists, because some of those work and the structure is fine. Others don't. You get some of the top level ones. Some have the playlists beneath them. Some of those are full and some empty. Some of the top leve ones are empty. Some are missing, at all levels.
It announces the synchronisation of each playlist, but they're not there (they might be if you look in the file system but that's no use to anyone). Third sync, it duplicates a subset, but adds none of the missing ones. Fourth sync, it duplicates everything, but ignoring the structure completely, so hundreds of playlists with no structure.
Steps to reproduce the problem: Use a fresh, newly formatted SD card. Use latest version of MM (whatever that is). Reinstall MM for Android. Sync to android. That's it. Keep doing it and the sync for playlists will fail differently each time.
"Copying playlists" - says MM for windows. Very slowly, you'd think text files would be quicker. If you're lucky, some subset of the ones it announced that it was copying might appear on the phone. But even if you can see them in the phone's file manager, that doesn't mean they'll be available in MM on the phone. Some might. Some won't.
You can delete all playlists from within android, look at the phone via Windows, and you'll see an empty playlists folder but the 'playlists' part of the tree still shows playlists. No wonder it's confused. It has no idea what is on the device. Even the device has no idea what is on the device. Hours of syncing: nothing.
As for wifi sync, it doesn't work. You can authorise the device, but nothing will ever happen. "Searching" ... that's it.
Right, end of rant, switching to constructive mode:
Windows 10 64-bit, Fairphone 2, MM Gold 126.96.36.1995, MM for Android whatever was on Google Play yesterday (with Pro).
: There's no point in trying to sync playlists if there are any on the phone. You have to delete ALL of them from within the phone, then sync, then wait for a good long time - budget 90 minutes for sync and an hour waiting - to see them appear on the phone. Don't use or disconnect your phone in this time. I've had this work once. Settings include: "Organise playlists using subdirectory for each playlist type", "Force relative paths", "Use Linux folder separator", "Use Unicode". But none of these will solve the problems by itself. It might
be safe to disconnect the phone once the sync is done, but I wouldn't risk it.