You will have to think of AmiBroker shapes differently than you think of them in a drawing program such as Photoshop or Illustrator.
In AB, the anchor points are the date/price coordinates. In a drawing program, the anchors are screen coordinates. As a result, when you select a shape and look at Properties, you will see X/Y as date-time and price.
So when you zoom, the date-time/price coordinates remain the same. However, the angle as visible on the chart at that time will be different. Now, you may ask why does AB display an angle value when when it varies with the zoom? I do not have an answer to that.
As a corollary, there is no such thing as a circle or a square in AB as it would change with the zoom. In a normal drawing program, you would typically select the ellipse tool and press Shift or Ctrl to draw a circle; but this has no meaning in AB and is, therefore, not available.