I activated a playsound command playing a custom song (the song is more than a minute long). While it was playing I went back to the main menu and the sound stopped.
Then I went into the "saves" folder and deleted the world I played the sound in. As soon as I went back to Minecraft and entereing another world the music started playing again and didn't stop even when I went back to the main menu.

I used Minecraft in fullscreen mode.

Here is a video showing how exactly this bug behaves: https://youtu.be/xjvIFRUkl_A