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
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This is MC-35856
It also works if you are not playing in fullscreen.