Seems like it’s your code located at /auth/twitch/callback that isn’t do what you expect it to do.
It doesn’t seem like it’s a Nginx issue or a cloudflare issue. But a “your code” issue.
Since these are brownser steps cloudflare would block the browser the user is using which is isn’t
And you can see the cloudflare is not blocking the request getting to nginx.
You can see that nginx is getting the request.
So whatever is processing the request is not doing what it’s supposed to do and is redirecting to / after failing to process the ?code