Are you using images or a circle created in Animate? If you used circles in Animate they should not change. To cut your circle so you have only half of it showing, create a symbol. Set the circle so that half is out of its bounding box and then choose overflow hidden.
At first I tried using an SVG, but then realized that, of course the border would scale with the rest of the image.
So now I am using a circle created in Animate. In the timeline of the animation, it says that the border is 1px, but that is not how it appears in the Stage or Preview. It scales up as well.
I now see what I did wrong. I had converted the circle to a symbol prior to using it in an animation.
I should have created the animation first and then turned it into a symbol.
Makes sense. Glad you fugured it out.