This is a feature in Android where it has sticky players even after an app is closed from background. Here's how it works:

1. Open an app with support for the feature. Usually music players have this: Vanilla Music
2. Play any music
3. Exit the app. The music would still play.
4. Pause the music from the player in the drop down quick settings
5. Close the app from background
6. Player is still persistent in the drop down settings and can be unpaused to have the player without the app.

Why implement this feature? Because when mpv has a persistent player and that player is kept unpaused, it can be easy to pick up from where it left by just wearing wireless earbuds or headphones. The user doesn't have to worry whether the app is in background or was removed from background. Without persistent player, every time a headphone is worn, user would need to open mpv and run the playlist again.