There was a problem using iframe in Grafana 7.1.5

I had a strange situation integrating grafana with a reverse proxy
Browser successfully redirected to grafana’s home page when I enter my proxy address directly on the browser
But when I use iframe to integrate grafana, the interface doesn’t redirect and stays in the login url

url: http://localhost:9091/grafana/login?username=admin

html:

this is a iframe test

This is my nginx configuration
server {
listen 9091;
server_name localhost;
error_log logs/grafanaError.log debug;

    location / {
        root   html;
        index  index.html index.htm;
    }

	location /grafana/ {
		#proxy_set_header X-WEBAUTH-USER "zanebono";
		proxy_set_header X-WEBAUTH-USER $arg_username;
        proxy_pass http://localhost:3000/;
    }

}

grafana configuration
[auth.proxy]
enabled = true
header_name = X-WEBAUTH-USER
header_property = username
auto_sign_up = true
sync_ttl = 60
whitelist =
headers =

Read the auth proxy docs for details on what the setting below enables

enable_login_token = true

Any help is useful, thank you!