Creating a tween on a layer creates a new layer instead

I've got a graphic symbol on a layer with a keyframe at the start of the timeline. A few frames later I created a new keyframe, moved the symbol to the right, and created a classic tween on the frames between the two keyframes. When I do this, the symbol in the first keyframe gets removed from that layer and moved to a newly created layer directly below it, which holds the tween (with no second keyframe on this new layer, so the tween doesn't actually do anything). The symbol is still on the original layer only on the second keyframe. 

What am I doing wrong for Animate to be moving symbols and creating new layers instead of making the tween on the original layer? (I tried with a motion tween and it does the same thing)