It could be doubled faces, but I think most of the time is just because of the shape maybe? Sometimes asking Blender to fill such a big kinda complex shape with a single face is a big of an ask, it doesn’t know what to do. It’s the type of thing that gets you the Terminator movies, machines getting grumpy when they become self-aware. So as you did, just adding some extra geometry to help Blender out on how to fill the face helps it on figuring out how to fill it. And also we avoid a future Skynet situation.
