This path has a long history in larger project. I have cut it out and put here into its own document. Somewhere in the past, when I have been working on this path, I must have put a mask over it, and it has never forgotten that mask. If you look at the properties and appearance, there is no reason for it to look like this.
Update: I discovered how to fix it. I do "expand appearance". This gives me a group, and then I ungroup and I get three layers. I delete the problem layer and the path becomes normal. However, I don't understand what I just did.