When keying the
modulate property, this property path is used by the animation player:
You can change it manually to
Sprite2D:modulate:a to only key the
a component of
modulate (click on the name of the animation track to edit it).
Also, for exported values that have several components using bezier curves automatically creates one track per property.
However if you want to animate the color and the alpha of the same property in two different tracks you have to hack it by adding a
Sprite2D:modulate track where you animate RGB and a
Sprite2D:modulate:a below which overrides the alpha of the first track.