I did some tests but I can't reproduce this inconsistent behavior that you are describing.
You can try for yourself here.
*I did some modifications to the shader based on what I think you are trying to achieve, but it works the same regardless of the modifications.
Here's what it looks like when previewing in Chrome:
I left a transparent border in two sides of the images (right and bottom) so it's easy to spot the differences with and without a border. You can see in the image that only the sides with a transparent border are rendered correctly.
Also added sine behaviors and a rotate to see what effects non round positions could have. It's possible to see some oscillation on the non-transparent borders of the circle, but nothing on the square.
Everything works consistently here, and adding other effects doesn't seem to extend the bounding box. Adding a transparent border or extending the texture through the xml solves the problem on my PC.
Are you getting the same results?