How to "purge" (database) list of songs ?

I have the same exact issue. According to Garmin Express, no more music listed, but still 12.1MB displayed in the Music size in the bar at the bottom, and in the music library on the watch, it shows me songs which are not playing because they're not on the device.

Had the same issue on my ex. 5+. The only way to remove them is to reset the watch (factory reset)

Settings > System > Reset.  > Delete Data and Reset Settings

I have done this because i sold my watch and the mp3 list was gone

Have you tried using the Android File Transfer utility to see if something really exists? Or just use GE?

Thank you but I'd like to achieve that without resetting the entire wath. I have so many settings and apps...

I have the same case. Did you get around this without resetting your watch?

Unfortunately, I have already the issue

Using Android File Transfer I made a backup of all files before deleted them from the watches, and phantom melodies still remained in the list on the watch. After that, I made "Rest Config + Erase Date" on the watch. I uploaded all files (histories, logs, configs, and maps) back to the watch. And only after that manipulation, the phantom list was erased.

Yes of course. Me too, but it's the only stupid way of doing this. I didn't find a better way.

Exactly. I spent the half-day on solving the problem differently, but... 
¯\_(ツ)_/¯