Nginx Reverse proxy can only find one site out of 3

Update the behaviour of only being able to load one site at a time appears to be client side browser related - If I open all 3 sites in one browser window (one tab each) then we get the behaviour described below. If I use 3 separate browsers (I used Edge, Opera and Chrome) then all 3 sites load as expected. /Update

I have an Nginx reverse proxy setup to pass external traffic to one of 3 internal servers (Windows server 2019 running IIS 10) based on the url (this is for a testing environment). The nginx server and the webservers are all in the same vlan.

This has been working as intended until this morning when following a reboot of the web servers nginx can now only load one of the 3 sites at a time.

If I try to open all 3 sites, the first one loads as normal, the other 2 instantly give 404 errors. If I wait a few minutes and refresh one of the 404s it will load but the first site that was ok will now go to 404, same happens if the third site is loaded.

The Nginx config hasn't been changed and was loading all 3 sites at the same time prior to the reboots this morning.

I have tested the nginx conf with nginx -t which came back ok I have reloaded the nginx conf with sudo systemctl reload nginx - no change to behaviour I have restarted the nginx server - no change in behaviour

Browsing the web servers directly all 3 sites work as normal.

I can telnet on port 443 from nginx to all 3 web servers and vice versa.

Any suggestions as to why the behaviour may have changed?

The nginx config is as below (I have replaced the actual urls but the conf is otherwise unaltered):

user www-data;
worker_processes auto;
pid /run/nginx.pid;
include /etc/nginx/modules-enabled/*.conf;

events {
        worker_connections 1024;
        # multi_accept on;
}
#added by Ian to allow streaming
stream {
# To add additional websites and servers:
# Add the hostname and backend label to the map, then add the upstream blocks, as in the commented example below.
# ***Once you have updated the config you MUST reload nginx with the following command: sudo systemctl reload nginx
    map $ssl_preread_server_name $name {
   url.1.com                             GunServer;
   url.2.com                             CatServer;
   url.3.com                             BlueServer;
  # <hostname of website goes here>     <text label for the backend server goes here>;
  # example: mywebsite.com       myserver;
}

# Then create a new upstream block using the label set above as shown below
# upstream myserver {
        # server <IP:port>;
#}

upstream GunServer {
        server 10.25.2.12:443;
}

upstream CatServer {
        server 10.25.2.13:443;
}

upstream BlueServer {
        server 10.25.2.11:443;
}

server {
    listen      443;
    proxy_pass  $name;
    ssl_preread on;
}
}

# all settings below this line were from nginx example conf