Hi,
Doubled checked the action, it works. I think it's simply because it doesn't make sense for what you want to achieve. but the signed angle is right.
basically, when you do test, use the "y" for direction, it makes a lot more sense visually, since y is up. so as you move the target around the owner, and keep them both on the same XZ plane, you'll see the signed angle in action.
So if you are on a top down shooter, you can know the angle between the player and an enemy using "y" as a direction, provided the player as the y axis aligned with the top down depth ( world y most likely)
Bye,
Jean