If your domain is what you set as webserver.domain in pihole.toml (or env vars in a docker container), we already have an automatic redirection / -> /admin/, however, if your domain is different, we intentionally don't do this.
The reason is that some users use IP blocking mode which replies to your clients with the IP address of your Pi-hole when a page is blocked. If the auto-redirect would fire here, too, you'd see your Pi-hole's dashboard (or the login page if you have a password) in many places on websites where you'd normally have seen the ads. This doesn't seem desired behavior.
But maybe this is an issue with your reverse proxy, please tell us more about your particular configuration:
- Have you tried setting
webserver.domain? - Are you hosting Pi-hole's dashboard on an IP address, a subdomain or maybe a path?
- ... (more interesting points about your reverse proxy setup)