Beginner Question: Can't remove outline from rectangle?

Hey there, I’m fairly new to this.
I’ve animated a simple name box for a video that pops up and then disappears. It’s just white rectangles on an alpha background. However, the rendered animation has a dark margin around the rectangle which I just can’t figure out how to remove. Outline width is set to 0, fill color and outline color are all set to white. It’s probably just a silly thing I missed, can anyone help?

Could you share a screenshot so we can see it?

You could delete the outline layer if you’ve no purpose with that.

On the layers dock panel, you’ll notice that the rectangle outline looks like a spline icon just above the rectangle layer.