No you start and stop the animation player. In case of play you supply the name of the track to queue up. But for stopping the animation player already is playing whatever it is playing. The animation player is what is playing the track it isn't a master controller that spawns a player to play the track. It makes sense and works how it should. I think you just likely misunderstand how it works.
Think of it like a cassette player. When you want to start it you also have to supply a cassette to play, when you stop you just stop it. If you call play again without supplying another cassette to play it just starts playing the same one again. If you call play with another name, then it changes the track.
https://docs.godotengine.org/en/stable/classes/class_animationplayer.html#class-animationplayer-method-play
The AnimationPlayer keeps track of its current or last played animation with assigned_animation. If this method is called with that same animation name, or with no name parameter, the assigned animation will resume playing if it was paused, or restart if it was stopped (see stop for both pause and stop). If the animation was already playing, it will keep playing.
