Azure Static website react.js getting 404 in the browser but returning the correct content on a path

So like I normally setup static websites, with error page going back to index.html. The browser response shows a 404 though, on local it works ok and shows a 200. I'm not sure if this is an issue with the reactjs router or the azure static website configuration. Anyway to send this back as a 200
in my console shows

GET https://app.testsite.com/signup 404
Navigated to https://app.testsite.com/signup

and then works ok. Some browsers have issue with this as it shows red in the console