No longer able to debug flutter web app behind reverse proxy due to "Unexpected sender"

I'm trying to develop a flutter web app behind a nginx server. It worked few weeks ago but now I got this notification :

https://github.com/dart-lang/webdev/blob/51b5484348b4a8ede351e8dff0428b083495ba78/dwds/debug_extension_mv3/web/messaging.dart#L212-L215

My nginx conf
```
server {
server_name
location /api {
proxy_pass
}

location / {
proxy_pass http://localhost:9020;

# FOR DEVELOPMENT
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "Upgrade";
}
}

```

The 9020 port is a remote docker container with the flutter app

Start with dwds/debug_extension_mv3/web/messaging.dart at lines 212-215, then reproduce the Flutter web app running from the remote Docker container on port 9020 through the provided nginx reverse-proxy configuration. Compare debugging behavior with and without the proxy and trace the conditions producing “Unexpected sender.” Done means Flutter web debugging works behind the proxy without that notification.

Written by the indexing model from the issue text.