I have a fairly similar issue.
At one point the playlists (m3u) on my USB flash drive would just disappear, but the music folder was still there, so I was at least able to play the music at random.
Then I read on here that Android may delete m3u`s if there isn`t a .nomedia file in the directory. So I created that and copied my playlist onto the USB drive again. Plugged them into the car`s USB port and on my Android 8 Headunit with Mediamonkey selected under sources USB and "Check for new/changed media".
However, no effect, apart from even the nusic now not showing up in Mediamonkey music folder anymore.
The music and the playlists show in Android file browser, the USB drive is definitely recognized by the system and they do work in VLC player app, but with a huge delay after starting and loosing the position within the playlist after I restart the car (which Mediamonkey preserved nicely, when it worked in the beginning).
I have dried (via App settings in Android) to wipe cache and data for the Mediamonkey app and then retry - no joy.
Removed the .nomedia file - no joy.
I have tried uninstalling Mediamonkey, deleting the file it created on the internal storage of the headunit, also. Then installing latest version available in Play-Store, retried - still doesn`t show me any playlists or music files, even though they pop up in other sources.
Anybody any ideas, apart from a full wipe of my Android Headunit???
Help would be greatly appreciated,
Many thanks in advance,