Have you modified the code in any way? Does this occur when you drop it into a new project? I'm afraid I'm unable to reproduce this on 8.3.7. Can you also confirm you have v1.2 of the outline shader? (it will say so at the top of the outline_shader.rpy file)
That's good to know! Unfortunately it does mean that it's interacting with something in your project then, which I can't debug. Note that shaders can interact with each other in odd ways sometimes, in case that gives you an idea of where to look. If you find any more information, let me know in case there are compatibility changes I can make!