That’s because you are not seeing the underlying vector art but the antialiasing.
Send a screenshot of how the cutter is being used, it may suggest a solution,
otherwise contact support to have a look at the project.
This works but its not the best solution in the world. I found that if you plug it several times into a composite then into your main composite it works but is there another way to get around this or are we doing something wrong?
A method I have seen used - I never quite understood why, until I read this question! - is to plug the cutter node into the composite node multiple times. I’m not sure why it works, but it does! I would have the render view on in the camera view, and in another panel, have the node view on. Keep adding additional connections from the cutter to the composite until the anti-aliasing line goes away.
To keep your network tidy, you can use an additional composite node to hold the multiple connections, and then connect that one to the main composite. Set the additional composite node’s render mode ‘pass-through’.