Alexa can misbehave like that from time to time when a simple "stop" or "stop the music" doesn't work - that's why I have that Silence routine. Another workaround is to switch away from the currently streaming tracks and switch to playing a Tunein radio station, then ask it to stop which seems to work better - that's just a little more difficult in a routine, but adding a duration to the play radio command can help mitigate that.