I see you are writing index.html instead of index.md in the about folder. Might be a typo or a probable reason for your issue. I too would expect layouts/about/single.html to overwrite layouts/_default/single.html. The problem is, you might have another template that is rendered and not overwritten in place of the _default one. If you post a link to a public repo we can have a look into this.
If you prefer to not dive into the issue you could have a look at the layout parameter for your frontmatter: