For Android 10 the internal addressing has been changed, so there are some manual steps needed to make them work again.1.
Connect your phone to the PC.2.
On your phone, click and hold the Play Music app (if you have that one installed), and click info. Force close the app.3.
Synchronize your playlists to the phone with MusicBee, choose M3U as playlist format.4.
In the meanwhile download and install NotePad++ on your PC.5.
Once the syncing is done, copy over the playlists from your phone (usually in /Internal Storage/Playlists) to your PC.6.
On your PC, open the playlist you want to fix in NotePad++.7.
Push the "Ctrl" + "F" buttons on your keyboard.8.
Here replace all the following instances of the stated path to prefix the string "/storage/emulated/0":9.
Safe the file.10.
Copy over the M3U file to the original on the phone and overwrite it.11.
If done right, your playlist will once again be recognized when you open your favourite music player. It might take a short while before the app has once again scanned the folder, so be patient.