I'm experiencing the same issue on my Mi 9 running 21.4.29. When stopping playback from the Spotify notification and then swiping on it left or right to remove it, it ends up like
@gerryrigs said. The notification can even be swiped back in. Not sure if a Spotify problem or a ROM problem. Killing Spotify removes the "blank notification" but that's not how it is supposed to work.